* Disable fallthrough attributes for the Intel compilers
On MacOS and Linux the Intel compilers may be identified as the
host compilers (Clang or GNU) but do not support the corresponding
compiler specific fallthrough attributes.
* Rearrange ifdef logic for excluding pre-C++17 fallthrough attributes
This puts Intel and PGI compilers into a separate group
and thus makes the intent and logic more obvious.
Axel Kohlmeyer