Remove dependency on the DLL of pythonxx.def file.

Remove dependency on the DLL of pythonxx.def file.

This confused Cygwin's make because of the colon in the path. The
DLL isn't likely to change under us so preserving the dependency
doesn't gain us much, and it's useful to be able to do a native
Windows build with the Cygwin mingw toolset.

Noah Misch.