Disable fallthrough attributes for the Intel compilers on Linux and MacOS (#1879)

* 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.

include/fmt/format.h

@@ -70,9 +70,11 @@
 #endif
 
 #if __cplusplus == 201103L || __cplusplus == 201402L
-#  if defined(__clang__)
+#  if defined(__INTEL_COMPILER) || defined(__PGI)
+#    define FMT_FALLTHROUGH
+#  elif defined(__clang__)
 #    define FMT_FALLTHROUGH [[clang::fallthrough]]
-#  elif FMT_GCC_VERSION >= 700 && !defined(__PGI) && \
+#  elif FMT_GCC_VERSION >= 700 && \
       (!defined(__EDG_VERSION__) || __EDG_VERSION__ >= 520)
 #    define FMT_FALLTHROUGH [[gnu::fallthrough]]
 #  else