Add a flag to Mac build to eliminate an unnecessary build warning.

Makefile.mac

@@ -6,7 +6,7 @@ DEFS=
 LIBS=-lm
 
 # Uncomment for a release optimized universal binary build
-CFLAGS=-arch i386 -arch x86_64 -Wall -O4 -pthread -mmacosx-version-min=10.6 -DNDEBUG $(INCLUDES) $(DEFS)
+CFLAGS=-arch i386 -arch x86_64 -Wall -O4 -pthread -mmacosx-version-min=10.6 -DNDEBUG -Wno-unused-private-field $(INCLUDES) $(DEFS)
 STRIP=strip
 
 # Uncomment for a debug build