pkg/lua: Suppress warnings on discarded qualifiers.

Upstream lua has many instances of discarding cont qualifiers through
explicit casting. This disables the warnigns for the package so that
they don't show up when one enables -Wno-cast-qual globally.

pkg/lua/Makefile.lua

@@ -6,6 +6,11 @@ endif
 
 CFLAGS += -DLUA_MAXCAPTURES=16 -DL_MAXLENNUM=50
 
+# Upstream has several of these warnings which will not be fixed: because it
+# would require a big refactor and because the lua developers do not accept
+# patches.
+CFLAGS += -Wno-cast-qual
+
 #    Enable these options to debug stack usage
 #          -Wstack-usage=128 -Wno-error=stack-usage=128