Maint: gnulib: Update how gnulib is built and linked against.

Instead of md5.c being built through a custom recipe in
lib/common/Makefile.am, use gnulib's own recipes.

This was originally written by Jan Pokorný as part of
https://github.com/ClusterLabs/pacemaker/pull/1756 but I've made a
couple changes.

• The gnulib updating to pick up the various other Makefile.am and m4 changes is going to happen in a separate commit.

• We cannot drop 00gnulib.m4. This file appears to accumulate fixes that will eventually make their way into autoconf. It grows and shrinks over time. At the moment, it contains fixes for references to gl_COMPILER_CLANG that we'll need.

• Keep the /lib/gnu/libgnu.a ignore line in the "Formerly built" section.

• Don't do the separate dependency for maint/gnulib/.git. We are always only running gnulib-update manually at release time, so there's no need for the extra rule complication.