build: Use gnulib-tool with a module list.

* .gitmodules: New file.
* gitsub.sh: New file, from gnulib/top/gitsub.sh.
* autopull.sh: Use gitsub.sh to check out gnulib.
* autogen.sh: Accept --skip-gnulib option. Invoke gnulib-tool, assuming
a gnulib checkout is already present.
* Makefile.devel (lib/aclocal.m4): Update reference to macros directory.
* lib/configure.ac: Likewise. Invoke gl_EARLY, gl_INIT.
* lib/Makefile.am (EXTRA_DIST, MOSTLYCLEANFILES): New variables.
Include Makefile.gnulib.

lib/.gitignore

@@ -1,4 +1,7 @@
 # Files brought in by gnulib-tool:
+/gnulib-m4/
+/Makefile.gnulib
+/dummy.c
 /filename.h
 
 # Files generated by the autotools: