clang-10: ignore fallthrough warning in generated code
Clang-10 will not recognize '/* fall through */' as an indicator to
turn off the fallthrough warning due to the lack of a 'break' in a case
statement.
Code generated by flex uses the '/* fall through */' comments to turn
off compiler warnings for fallthroughs in case statements.
For code generated by flex, ignore the implicit-fallthrough via pragma
or disable the warning via a compile time flag.
Add new env variable "CFLAGS_NOIMPLICIT_FALLTHROUGH" to selectively
disable the compile check in Makefiles when checking is enabled.
Change-Id: I4c054defda03daa2aeb645ae2271dfa0cb54925f
Reviewed-on: https://gerrit.openafs.org/14275
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>