cpp, qt: Handle modified includedir installation

* lang/cpp/src/Makefile.am,
lang/qt/src/Makefile.am: Replace resolved_includedir.
* lang/cpp/src/GpgmeppConfig.cmake.in.in,
lang/qt/src/QGpgmeConfig.cmake.in.in: Use resolved_includedir
instead of relying on a common installation prefix.

This fixes usage of the config files in case gpgme
is configured with a custom --includedir.
We have to do the replacement manually like for libdir
because configure would include variables in the replacement.