Fix lua check to work on opensuse leap 42

Bug 3815

It would be nice to figure out a way to remove the check for version all
together but I (Danny) couldn't figure out how that would be done since we
need to know which libs/headers to use and on systems with multiple
installed and no 'lua' lib (Ubuntu) you have to use the PKG_CHECK_EXISTS
to set up the pkg name for PKG_CHECK_MODULES or you don't get things set up
correct when trying to link.