ok most already know this but i'll post anyway, and yes i know the build script says its for 1.25.
if you remove
# This is from the slackbuild of Vincenzi Loris(email@example.com)
sed -i '/[2de].M/d' configure.in
echo "AC_OUTPUT(common/Makefile common/mp4v2/Makefile libfaac/Makefile frontend/Makefile include/Makefile Makefile)" >> configure.
it will compile nicely
have sent an email to slackbuilds so hopefuly the change will be made soon