build: Generate and install pkg-config file with cmake
* src/CMakeLists.txt: Generate and install gpgmepp.pc.
* src/gpgmepp.pc.in: Use cmake variables. Use ${prefix}/${exec_prefix}
for exec_prefix, includedir, libdir. Remove host. Use fixed values for
Cflags and Libs.- GnuPG-bug-id: T7262