Fix/Improve static and shared builds with CLN or Poly (#7306)

Fix/Improve static and shared builds with CLN or Poly (#7306)

We already created two dependency targets `GMP_SHARED` and `GMP_STATIC`, as we could not use `libgmp.a` for shared linking (as it is built without `-fPIC`). This PR fixes our handling of CLN and Poly: they would always link with `GMP_STATIC`, leading to having both `GMP_SHARED` and `GMP_STATIC` in the linker command line in certain situations. We now also have `*_SHARED` and `*_STATIC` for both CLN and Poly.