Explicitly specify zlib include directory

This is particularly important when building python against a custom
version of zlib. It prevents issue when zlib is built with a custom
mangling prefix.

Thanks to Hans Johnson for identifying and fixing the problem.

See #15
Use "ZLIB_LIBRARY" instead of "ZLIB_LIBRARIES" as expected by FindZLIB

See #17
Merge branch '15-consider-zlib-include-dirs'

* 15-consider-zlib-include-dirs:
  Use "ZLIB_LIBRARY" instead of "ZLIB_LIBRARIES" as expected by FindZLIB
  Explicitly specify zlib include directory