Be more picky about Mac OS X linker warnings

...useful for cases like 913ba61d "Make liborcus
-fvisibility setting consistent with LO one."

Feel free to revert if it causes trouble, though.  (I was only able to test it
with a 10.7 build, not a 10.6 baseline one.)

Change-Id: Ic8e57cd917ef186f4dec8d876fc8da6d6db57b3d

solenv/gbuild/platform/macosx.mk

@@ -91,6 +91,7 @@ gb_OBJCXXFLAGS := -x objective-c++ -fobjc-exceptions
 gb_OBJCFLAGS := -x objective-c -fobjc-exceptions
 
 gb_LinkTarget_LDFLAGS := \
+	-Wl,-fatal_warnings \
 	$(SOLARLIB) \
 #man ld says: obsolete	-Wl,-multiply_defined,suppress \