lua: fix logic to enable lua support in configure

When there is no --enable-lua or --with-lua-pc, Lua should not be
enabled.

This fixes a bug introduced with 12e93188 (configure/makefile:
Allow specify Lua pkg-config file with --with-lua-pc) that caused
configure script to fail if lua headers was missing.

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>