m4: update ax_check_compile_flag.m4 from autoconf-archive

m4: update ax_check_compile_flag.m4 from autoconf-archive

I noticed this when doing some M4 macro analysis work, and saw
that the version of `ax_check_compile_flag.m4` in Tor has two serial
lines ('serial 5' and 'serial 6') which is invalid.

We could just fix one of the lines, but it makes more sense to just
sync with upstream, I think.

Import ax_check_compile_flag.m4 from autoconf-archive at latest
commit at time of writing (4e8aab846b0872fba99f1fe02ebcdff178a34c87).

Signed-off-by: Sam James <sam@gentoo.org>