gnu: Add luajit

* gnu/packages/lua.scm (luajit): New variable.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

Merge branch 'master' into core-updates

Conflicts:
	Makefile.am
	gnu/packages/base.scm

gnu: Add lua.

* gnu/packages/lua.scm: New file.
* Makefile.am: add it.