Revert to -O3 with Clang

Instead of -O4 option that does not work with both mingw and
Linux gcc (tested with Clang 3.1).

As reported by Reed Kotler:
Turns out that -O4 is not a valid option for clang unless you have
the proper gold linker and plugins built. That's because -O4 enables
LTO, which writes out bitcode files during the compile, and then loads
those and optimizes them during the link phase.

It requires a linker that supports LLVM's LTO. There is a plugin for
Gold available as part of LLVM.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/Makefile

@@ -270,7 +270,8 @@ ifeq ($(optimize),yes)
 	endif
 
 	ifeq ($(comp),clang)
-		CXXFLAGS += -O4
+		### -O4 requires a linker that supports LLVM's LTO
+		CXXFLAGS += -O3
 
 		ifeq ($(os),osx)
 			ifeq ($(arch),i386)