Allow CFLAGS from the system.

Author: sergiobenrocha2

Makefile

@@ -569,7 +569,7 @@ DEFS += -DLUA_COMPAT_APIINTCASTS
 
 # CFLAGS is defined based on C or C++ targets
 # (remember, expansion only happens when used, so doing it here is ok)
-CFLAGS = $(CCOMFLAGS) $(CPPONLYFLAGS)
+CFLAGS += $(CCOMFLAGS) $(CPPONLYFLAGS)
 
 # we compile C-only to C89 standard with GNU extensions
 # we compile C++ code to C++98 standard with GNU extensions