Refer to avx512er and avx12pf only for gcc < 15

gcc 14 warns that support for avx512er and avx12pf will be removed in gcc 15.

Here I'm making sure the compilation with gcc 15 will not fail, but I have the feeling we need a better way to fine tune how we compile TestVDTMath.cpp, taking into account the differences between all the compilers we support.

This also removes support for gcc < 8.