Assume gcc is at least 6.4, the oldest xtoolchain in the ports tree.
Assume clang is at least 6, which was in 11.2-RELEASE. Drop conditions
for older compilers.
Reviewed by: imp (earlier version), emaste, jhb
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D24802
Detect ancient GCC specifically, rather than using target architecture as a
crude heuristic.
Side note: compilers should really ignore -Wno- and -Wno-error= flags they
don't recognize. Seems like modern compilers produce warnings instead of
errors. Though, with -Werror they turn into errors. Clang's error can be
disabled with -Wno-error=unknown-warning-option, but GCC doesn't seem to
have a named method to disable the specific warning.
Submitted by: rpokala@ (earlier version)
Suggested by: rpokala@
Reviewed by: tinderbox
Sponsored by: Dell EMC Isilon
Update function definitions to post-ISO style; remove deprecated "register"
keyword. Correct indentation. Sprinkle 'static' on file-local variables.
Appease warnings at WARNS=6, for both Clang and GCC.
Sponsored by: Dell EMC Isilon
