README: mention CFLAGS on *nix builds

README

@@ -10,7 +10,7 @@ Dependencies:
 
 Basic *nix build instructions:
 	./autogen.sh
-	./configure
+	CFLAGS="-O3 -Wall -msse2" ./configure
 	make
 
 Basic WIN32 build instructions (on Fedora 13; requires mingw32):