CMake: Add support for libintl

CMake: Add support for libintl

This basically just links everything to libintl if USE_NLS is
on. It would be better to just link those targets that are
actually translated, but doing so is a huge PITA.

Also move the include_directories() for the build-tree include/
directory to the top of the CMakeLists.txt, otherwise it only
gets passed after Intl_INCLUDE_DIRS, which means we will built
against installed apt-pkg headers (if any) instead of our own.

Gbp-Dch: ignore