When source files are given in the form:

c:\foo\bar.c

the object file is written to:

c:\foo\bar.o

instead of the build\temp.win32-2.3 directory.  This is 
because the object_filenames method doesn't chop off 
the drive name from source file name base before trying 
to concatenate the build directory with the base.  This 
results in the build directory being ignored.  The 
ccompiler.py object_filenames does it right, so I just 
copied its behavior.