Build: GNUmakefile: refresh gnulib modules fetching procedure

We also move the default checkout directory of Gnulib (used
upon "make gnulib-update") to maint/gnulib, and expressly mark
that directory in .gitignore. One follow-up idea would be to
mount the Gnulib tree as a git submodule there, to explicitly
track its version supported (blessed) by pacemaker (though
it's just a possible next step upon further deliberation).