Add $(CFLAGS) to the simplified build rule for .so libraries on Darwin.

Arguably we should do this on *all* platforms, but for the moment I'll be
conservative and just do it where it's demonstrably needed.  Per report
from Brian Wipf.

src/makefiles/Makefile.darwin

@@ -13,6 +13,6 @@ endif
 # Rule for building shared libs (currently used only for regression test
 # shlib ... should go away, since this is not really enough knowledge)
 %.so: %.o
-	$(CC) -bundle -o $@ $< $(BE_DLLLIBS)
+	$(CC) $(CFLAGS) -bundle -o $@ $< $(BE_DLLLIBS)
 
 sqlmansect = 7