Add management of `/lib/cpp`

[ikeycode]

As a (nasty) hack right now we force cpp to be the one coming from gcc.

However, we should actually provide /lib/cpp (at %(libdir)/cpp with usr-merge) to automatically switch between clang-cpp (LLVM) and cpp (GCC)

[ikeycode]

This has become more important so that we provide clang-cpp link when CPP isn't respected and /lib/cpp is blindly queried by old autotools cruft.