kbuild: Disable -Wunused-but-set-variable for gcc 4.6.0

Disable the new -Wunused-but-set-variable that was added in gcc 4.6.0
It produces more false positives than useful warnings.

This can still be enabled using W=1

Signed-off-by: Dave Jones <davej@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Tested-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>

Makefile

@@ -567,6 +567,10 @@ ifndef CONFIG_CC_STACKPROTECTOR
 KBUILD_CFLAGS += $(call cc-option, -fno-stack-protector)
 endif
 
+# This warning generated too much noise in a regular build.
+# Use make W=1 to enable this warning (see scripts/Makefile.build)
+KBUILD_CFLAGS += $(call cc-option, -Wno-unused-but-set-variable)
+
 ifdef CONFIG_FRAME_POINTER
 KBUILD_CFLAGS	+= -fno-omit-frame-pointer -fno-optimize-sibling-calls
 else

scripts/Makefile.build

@@ -66,6 +66,7 @@ warning-1 += -Wmissing-format-attribute
 warning-1 += -Wmissing-prototypes
 warning-1 += -Wold-style-definition
 warning-1 += $(call cc-option, -Wmissing-include-dirs)
+warning-1 += $(call cc-option, -Wunused-but-set-variable)
 
 warning-2 := -Waggregate-return
 warning-2 += -Wcast-align