It appears various symbols are missing from Python27.lib, 64bit build.
Here is a list of undefined symbols (from link errors):

/debug/Opcodes && /c/mingw-builds/x64-4.8.1-posix-seh-rev5/mingw64/bin/gcc.exe -Wl,--add-stdcall-alias -shared -o ../py.dll -Wl,--major-image-version,0,--minor-image-version,0 -Wl,--whole-archive CMakeFiles/py.dir/objects.a -Wl,--no-whole-archive /C/Python27/libs/python27.lib -lkernel32 -luser32 -lgdi32 -lwinspool
-lshell32 -lole32 -loleaut32 -luuid -lcomdlg32 -ladvapi32
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x14a):
undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x973):
undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x99d):
undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0xa83):
undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0xc23):
undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0xc4b):
more undefined references to `__imp_PyExc_RuntimeError' follow
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x1b22):
 undefined reference to `__imp__Py_NoneStruct'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x1b7c):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x1d19):
 undefined reference to `__imp_PyFloat_Type'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x1d6d):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x205c):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x21f5):
 undefined reference to `__imp__Py_NoneStruct'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x2251):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x2388):
 undefined reference to `__imp_PyFloat_Type'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x2404):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x263c):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x291c):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x2a33):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x2b23):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x2bc0):
 undefined reference to `__imp_PyFloat_Type'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x2c44):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x2e4b):
 undefined reference to `__imp__Py_NoneStruct'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x2ece):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x312c):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x340c):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x36ac):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x38e9):
 undefined reference to `__imp__Py_NoneStruct'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x3941):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x3cfe):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x40ac):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x431c):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x45ac):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x486c):
 more undefined references to `__imp_PyExc_RuntimeError' follow
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x6c99):
 undefined reference to `__imp_PyFloat_Type'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x6cdd):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x6e03):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x7093):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x72cc):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x754c):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x77dc):
 more undefined references to `__imp_PyExc_RuntimeError' follow
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x7c3b):
 undefined reference to `__imp_PyFloat_Type'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x7c7f):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x7d7c):
 undefined reference to `__imp_PyFloat_Type'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x7df4):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x806c):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x8173):
 undefined reference to `__imp_PyFloat_Type'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x81e4):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x82b3):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x84b9):
 undefined reference to `__imp__Py_NoneStruct'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x8511):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x85e2):
 undefined reference to `__imp_PyFloat_Type'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x8664):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x891c):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x8acb):
 undefined reference to `__imp_PyFloat_Type'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x8b1f):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x8c93):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x8df3):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x8f03):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x8fe3):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x9163):
 more undefined references to `__imp_PyExc_RuntimeError' follow
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x95c6):
 undefined reference to `__imp_PyFloat_Type'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x960c):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x987c):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x9bcc):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0x9eac):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0xa14c):
 undefined reference to `__imp_PyExc_RuntimeError'
CMakeFiles/py.dir/objects.a(pythonopcodes.c.obj):pythonopcodes.c:(.text+0xa3fc):
 more undefined references to `__imp_PyExc_RuntimeError' follow

These symbols do not exist in 64bit, but are there in the 32bit library.See for instance PyFloat_Type:

victor@ligetipc ~/csound6/debug
$ nm /c/Python27/libs/Python27.lib | grep "PyFloat_Type"

victor@ligetipc ~/csound6/debug
$ nm /c/Python27-i386/libs/libpython27.a | grep "PyFloat_Type"
00000000 I __imp__PyFloat_Type
00000000 T _PyFloat_Type