maint: don't forget to distribute GNUmakefile.

* Makeflie.am (EXTRA_DIST): Add GNUmakefile.
maint: mkrockspecs simplifications.

* luarocks-config.lua.in: Remove.
* configure.ac (AC_CONFIG_FILES): Remove luarocks-config.lua.
* GNUmakefile (luarocks-config.lua): Make on demand.
* Makefile.am (rockspecs): Move from here...
* GNUmakefile (rockspecs): ...to here.
(WOGER_ENV, WOGER_OUT): Factored out of release rule.
(release): Simplify accordingly.