m4: discard stderr, in case Lua fails for any reason.

* m4/ax_lua.m4 (_AX_LUA_CHK_VER): For compatibility with the contract
from the previous (shell|sed|awk) implementation, discard stderr output.
(AX_PROG_LUA): Missing backtick.

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