maint: use a git submodule for gnulib

* .gitmodules: New file, to track gnulib.
* .gnulib: Submodule directory.
* Makefile.am (EXTRA_DIST): Don't list config.rpath or
gitlog-to-changelog.
* autogen.sh: Adapt to use the new submodule.
* cfg.mk: New file.
(SUBDIRS): Add gnulib/lib and gnulib/tests.
(dist-hook): Reflect new location of getlog-to-changelog.
* configure.ac: Set build-aux/ as AUX_DIR.
Invoke gl_EARLY and gl_INIT.
(AC_CONFIG_FILES): Add gnulib/lib/Makefile and gnulib/tests/Makefile.