* .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.
* configure.ac: Invoke AC_INIT, AC_OUTPUT without arguments.
* doc/configure.ac: Likewise.
* lib/configure.ac: Likewise.
* src/configure.ac: Invoke AC_INIT, AC_OUTPUT without arguments. Use
AC_CACHE_CHECK, AC_LANG_PUSH/AC_LANG_POP, AC_COMPILE_IFELSE
* tests/configure.ac: Invoke AC_INIT, AC_OUTPUT without arguments. Use
ac_cv_c_compiler_gnu instead of ac_cv_prog_gcc, and
ac_cv_cxx_compiler_gnu instead of ac_cv_prog_gxx. Don't use 'test' with
option '-a'.