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

This commit will prevent the following error from occurring when
configuring using the default option where no zlib is installed on
the system:

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

See #15
Merge branch '15-consider-zlib-include-dirs'

* 15-consider-zlib-include-dirs:
  Fix "binascii" configure error happening when no zlib is found