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/Makefile.am

@@ -17,12 +17,18 @@
 ## Process this file with automake to produce Makefile.in.
 
 AUTOMAKE_OPTIONS = 1.11 foreign
+EXTRA_DIST =
+MOSTLYCLEANFILES = core *.stackdump
 
 noinst_LIBRARIES = libgp.a
-libgp_a_SOURCES = \
+
+# A small set of gnulib modules.
+include Makefile.gnulib
+
+libgp_a_SOURCES += \
   getopt.h getopt.c getopt1.c \
   getline.h getline.cc \
   hash.h hash.cc
 
-EXTRA_DIST = \
-  filename.h
+# Allow users to use "gnulib-tool --update".
+EXTRA_DIST += gnulib-m4/gnulib-cache.m4