Fixed redefinition error when compiling with g++12

[sprenger120]

Two tests won't compile when done so with g++12 because of an old workaround added for MSVC. I've extended it to include g++ and just plucked out the __cpluscplus version string from my locally installed filesystem include file.

[johnmcfarlane]

Thanks @sprenger120! Note, I've bypassed the CI as it's likely not passing currently due to bit rot.