maint: pacify 'make dist'

* src/dfa.c (parse_bracket_exp): Reindent with spaces.
* src/dfa.h (case_folded_counterparts): Prefix decl with 'extern'.
* src/main.c: Don't include assert.h.
tests: make a performance-measuring test less system-sensitive

Andreas Schwab reported in http://debbugs.gnu.org/16941
that this test would timeout and fail on m68k-suse-linux.
Rather than testing absolute duration with a limit tuned
to today's hardware, compare performance of grep with LC_ALL=C
against that same command using LC_ALL=ja_JP.eucJP.
* tests/init.cfg (require_hi_res_time_): New function.
* tests/mb-non-UTF8-performance: Rewrite to use it:
record absolute duration D of the first (normally much faster)
command, and set a timeout of 8*D for the command running in
an affected locale.