ScalES-PPM

The scales-ppm{,-core}.pc.in pkg-config files specifies several libraries in the Libs: portion which are actually internal, private library dependencies of scales-ppm. That is, they are not part of the public interface/API and are just implementation details. These should be specified instead in Libs.private: so that they are only added to the link when creating a static library.

Similarly, the MPI includes are part of the public scales-ppm-core API, so we need to include those, but we don't need parmetis or metis includes here (in any case, they should have been in scales-ppm.pc.in, not scales-ppm-core.pc.in, but they're not needed there either since they're again not part of the public API).

See attached patch and [1-2] for how libraries should be specified for linking.

[1] https://people.freedesktop.org/~dbn/pkg-config-guide.html
[2] https://cmake.org/pipermail/cmake/2016-May/063400.html