do not provide clang symlinks unless clang is installed

The idea to install clang/clang++ symlinks unconditionally is flawed (see the commit). It for example breaks 'osc build llvm.spec --icecream=40', because llvm's configure first checks CC/CXX, finds them empty, then it prefers clang to gcc, so checks for clang first, "finds" it because of the symlink, but then further checks because this "compiler" does not work.

Strictly technically speaking, gcc symlinks should be treated the same way, but there breakages seem less likely, so I'd wait until somebody complains (and I'm also not sure how to handle the cc/c++ symlinks, as not systems where gcc symlinks would need to be separate cc/c++ would presumably not point to gcc/g++ either).