Systems installing both KF5 gpgmepp (or KDEPIMLibs 4) run into header file
confusion or conflict when also installing the C++ wrappers from gpgme.

To avoid most cases of confusion (where headers are included from include/gpgme++
instead of include/KF5/gpgme++) gpgme can be built to install its headers in a
private location like include/gpgme, using --includedir=${prefix}/include/gpgme.

This works, but currently requires manual editing of Gpgmeppconfig.cmake because
the default include directory is hardcoded in its template file. I think that's a
bug.