There were several issues caused by travis-ci using cmake-3.2.2
instead of what I have locally, cmake-3.7.2:
- FIND_PACKAGE(PkgConfig) sets PKG_CONFIG_FOUND, not PkgConfig_FOUND.
- Set cache internal variables instead of parent scope. (Probably not
needed, but it's a good idea.)
- Older PkgConfig CMake modules don't support PKG_GET_VARIABLE(), so use
defaults for Nautilus and Thunar extension directories in that case.
- Added diagnostic messages for older PkgConfig.
- Print the cmake version in travis-ci.
Squashed commit of the bugfix/travis-ci-gtk branch.
Rewrote FindGLib2, FindGTK3, FindGdkPixbuf2, FindLibNautilusExtension,
and FindThunarX2 to use FIND_LIBRARY_PKG_CONFIG().
Fixed CFLAGS_OTHER on ThunarX2. (Incorrect capitalization.)
