configury: clean up ldoc droppings with make maintainer-clean

* Makefile.am (MAINTAINERCLEANFILES): Add docs/index.html and
docs/ldoc.css.
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.
configury: update ax_lua.m4 to latest version, which simplifies configure.ac slightly.
Merge pull request #62 from gvvaughan/pull-request/remove-empty-lib-subdir

gnulib: no need to link always empty libgnu.
Merge pull request #61 from gvvaughan/pull-request/maint-clean-ldocs-output

configury: clean up ldoc droppings with make maintainer-clean
m4/ax_compare_version.m4: add support file for ax_lua.m4
lposix.c: Reuben Thomas is no longer the maintainer.
Use bit32 (now available for 5.1) rather than luabitop: slightly simplifies things.
Add isatty.
Add STD{IN,OUT,ERR}_FILENO constants.
tests-posix.lua: remove commented-out test for non-POSIX putenv function
Update ax_lua.m4 to latest: no code changes, formatting only.
configury: update for releasing on 'release' branch of github rather than via tarballs.
Bump version down to 5.1.24: we haven't made such a release yet
lposix.c: correct cross-reference to getopt.lua
tests-posix.lua: remove a debugging line.
configure.ac: remove useless AC_SUBST calls for precious variables.
Makefile.am: remove obsolete definition of ROCKSPEC
configury: add rockspec-generating files for git/release rockspecs.
rockspecs.lua: fix a misnamed variable