Fix "binascii" configure error happening when no zlib is found

This commit will prevent the following error from occuring:

CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
ZLIB_INCLUDE_DIR (ADVANCED)
   used as include directory in directory C:/D/W/CPython-64-VS2010/Continuous/CPython-64-cl-VS2010-Win64/cmake/extensions
ZLIB_LIBRARY (ADVANCED)
    linked by target "extension_binascii" in directory C:/D/W/CPython-64-VS2010/Continuous/CPython-64-cl-VS2010-Win64/cmake/extensions