Make TOOLS_CFLAGS more easily customisable in the Makefile.

Fixes Bugzilla #4155.

Makefile

@@ -1073,10 +1073,10 @@ makedirs:
 #############################################################################
 
 TOOLS_OPTIMIZE = -g -O2 -Wall -fno-strict-aliasing
-TOOLS_CFLAGS = $(TOOLS_OPTIMIZE) \
-               -DTEMPDIR=\"$(TEMPDIR)\" -DSYSTEM=\"\" \
-               -I$(Q3LCCSRCDIR) \
-               -I$(LBURGDIR)
+TOOLS_CFLAGS += $(TOOLS_OPTIMIZE) \
+                -DTEMPDIR=\"$(TEMPDIR)\" -DSYSTEM=\"\" \
+                -I$(Q3LCCSRCDIR) \
+                -I$(LBURGDIR)
 TOOLS_LIBS =
 TOOLS_LDFLAGS =