Adjust compiler flags for modern gcc

The gcc compiler has unused value warnings enabled by default, so add
-Wno-unused-value flag to reduce noise in the compiler output.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>

Makefile

@@ -20,7 +20,7 @@ SHELL = /bin/sh
 
 # The C compiler and linker to use
 CC	= gcc
-CFLAGS	= -g -O2 -Wall
+CFLAGS	= -g -O2 -Wall -Wno-unused-value
 LD	= gcc
 LDFLAGS =