Test C++17 standard library more robustly

Sometimes the std::filesystem library is in a separate library, which can now be found, even if it comes from libc++.

The existing code needs to be re-ordered so that if it is appropriate to look for the standard library (and then perhaps a separate filesystem library) the search for the filesystem library is done after handling GMX_GPLUSPLUS_PATH. GROMACS now checks that the compiler accepts --gcc-toolchain flag before using it.

Avoided the anti-pattern of re-using cache variables. Updated various comments. Removed support for the Intel classic compiler, as GROMACS already does not support that.

Closes #4718 (closed) Closes #4728 (closed)