gnulib: no need to link always empty libgnu.

Now that we have no C modules from gnulib, there's no need to
build and link the always empty library any more.
* Makefile.am (SUBDIRS): Remove.
* configure.ac (AC_CONFIG_FILES): Remove lib/Makefile.
* bootstrap.conf (gnulib_tool_options): Add --avoid=dummy to skip
the one remaining empty gnulib C source file.
(luaposix_remove_empty_lib): gnulib-tool now generates a useless
lib subdirectory containing just a Makefile.am and .gitignore. We
don't use or need it any more, so delete it.
(luaposix_finish): Don't copy dummy.c out of gnulib.