Better compiler detection
Configure.h can only state a machine is a one compiler.
It used to be possible for vtk-m to say it was multiple compilers, for example it could be both GCC and PGI, Clang and MSVC ( yes possible ), or Intel and Clang.
This logical restructure now makes that impossible, and instead prefers a system where we choose the most specialized version of the compiler over the most general, where general is GCC / Clang.