m4: don't use GNU grep extensions in portable scripts.
m4: don't use GNU grep extensions in portable scripts.

* m4/ax_lua.m4 (AX_PROG_LUA): \+ is not valid in standard BRE.
Use bare + in ERE with `grep -E`.

Signed-off-by: Gary V. Vaughan <f9023000f29773649f3850298becb9544b5fd6a9@gnu.org>

gvvaughan/slingshot