[Cmake] finding matching includes and libraries

[Brad King]

Hi Fred,

> IF(ZLIB_INCLUDE_DIR)
>   FIND_LIBRARY(ZLIB_LIBRARY z
>     ${ZLIB_INCLUDE_DIR}/../lib
>   )
> ENDIF(ZLIB_INCLUDE_DIR)
>
> I'm not sure about the portability of "${ZLIB_INCLUDE_DIR}/../lib".
> Perhaps there is a better way to strip off the "/include" from
> ZLIB_INCLUDE_DIR in CMake.

This is safe and works well.  Several other find modules do this already.
FindZLIB was one of the first modules, and hasn't been updated.

-Brad