On Windows I can't use a source build of Python to create a venv which will compile C extensions because pyconfig.h cannot be found.  For example

    running build
    running build_ext
    building 'demo' extension
    creating build
    creating build\temp.win32-3.3
    creating build\temp.win32-3.3\Release
    c:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\BIN\cl.exe /c /nologo /Ox /MD /W3 /GS- /DNDEBUG -IC:\Repos\cpython\include -IC:\Users\oudkerk\myenv\PC /Tcdemo.c /Fobuild\temp.win32-3.3\Release\demo.obj
demo.c
    C:\Repos\cpython\include\Python.h(8) : fatal error C1083: Cannot open include file: 'pyconfig.h': No such file or directory
    error: command '"c:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\BIN\cl.exe"' failed with exit status 2

The problem seems to be with the following line in distutils/command/build_ext.py:

            self.include_dirs.append(os.path.join(sys.exec_prefix, 'PC'))

Inside a venv, sys.exec_prefix is the venv directory.