Lua configure.ac improvements

Lua configure.ac improvements

Use PKG_WIRESHARK_CHECK_SYSTEM_MODULES for consistency with other
system dependencies.

Give slightly more informative configure messages other than multiple
'checking for LUA'.

Check for headers using pkg-config to avoid undefined symbols in
config.h.

Change-Id: I8d3df9ccf6f718d7d055da4f713af60d46d89e33
Reviewed-on: https://code.wireshark.org/review/14173
Petri-Dish: João Valverde <j@v6e.pt>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: João Valverde <j@v6e.pt>