Fixed problems with cmake files

Problems included:
 - Was using LIBHARU_* instead of LIBHPDF_*
 - Was using a add_definitions() hack for -DLIBHPDF_HAVE_NOPNGLIB,
   rather than using define flags in the generated config files.
 - Generated config files were not correctly importing the cmake variables.
 - Added extra HAVE_MEMORY_H and friend detection routines,
   to match what automake does.
Fix for cmake - summary printout wasn't working.
Fixes for cmake

Renamed some variables at the source of creation,
changed !ZLIB_FOUND to NOT ZLIB_FOUND
Add option to enable exceptions.
Added clang to list of compilers to ask about exceptions
Merge remote-tracking branch 'remotes/paulharris/bugfixes/export_and_cmake'