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.

Makefile.devel

@@ -7,7 +7,7 @@ MAKE = make
 all : lib/aclocal.m4 configures src/config.h.in lib/config.h.in lib/Makefile.in
 
 lib/aclocal.m4 : force
-	cd lib && aclocal -I ../m4
+	cd lib && aclocal -I gnulib-m4
 
 CONFIGURES = configure lib/configure src/configure tests/configure doc/configure