Add Autoconf archive macros to check for -Wall and its dependencies.
Adjust m4/.gitignore to only ignore libtool macros.
Require Autoconf 2.65 for compatibility with Autoconf archive macros.
Keeping separate files is more maintainable than merging them into a
single aclocal.m4 file. It is easier for users to understand where those
files come from.
Automake knows how to distribute files in the m4 directory.