build: Add option CONFIGURE_FOR_DISTRIBUTION
* CMakeLists.txt: Add option CONFIGURE_FOR_DISTRIBUTION. Set LibGpgError and Gpgme as required if option CONFIGURE_FOR_DISTRIBUTION is not set. Add src (and tests) only if required dependencies were found.
This makes it possible to create a release tarball of gpgmeqt without
first having to build the required version of gpgme, etc.