Problem: filetype: autoconf filetype not always correct
Solution: Detect aclocal.m4 as config filetype, detect configure.ac as
config filetype, fall back to POSIX m4 (Damien Lejay).
closes: #18065
Signed-off-by: Damien Lejay <damien@lejay.be>
Signed-off-by: Christian Brabandt <cb@256bit.org>