[CMake] Improve FindGMP (#8846)

[CMake] Improve FindGMP (#8846)

Fixes #8792. This commit fixes issues with/improves our current
implementation of FindGMP:

- Version check that does not rely on regex matches in `gmp.h`. Instead,
  this commit uses a test program that checks at compile-time whether
  the GMP version is recent enough. This is more robust, because on some
  systems (such as Fedora), `gmp.h` includes another file that has that
  version information.
- It now also checks for the `gmpxx.h` header and the `gmpxx` library.
- The commit changes the compile test to use the include directories and
  libraries that were found using `find_path` and `find_library`.