Reintroduce ACLOCAL_AMFLAGS to S3_0
Ever since we dropped ACLOCAL_AMFLAGS from main Makefile.am, autogen.sh has been giving warnings like "configure.ac:557: warning: macro 'AM_LANGINFO_CODESET' not found in library"
I think the warning comes from libtool, which last has had an upstream release 2015, and which doesn't know that automake doesn't need ACLOCAL_AMFLAGS any more. Situation is unlikely to change before we release 3.0.0, and in any case already released distributions have the libtool version that gives the warning. I think we should just reinstate ACLOCAL_AMFLAGS for S3_0.
Given that there's only some vague talk about developing a new libtool version that would drop the requirement, I think we should port this change to S3_1 too. Even if the new libtool version exist at the time of freeciv-3.1 release, there are going to be a lot of older distribution releases in use that do not have it.
-> consider 36h review period before pushing this to S3_1 started.