kbuild: move extra gcc checks to scripts/Makefile.extrawarn

W=... provides extra gcc checks.

Having such code in scripts/Makefile.build results in the same flags
being added to KBUILD_CFLAGS multiple times becuase
scripts/Makefile.build is invoked every time Kbuild descends into
the subdirectories.

Since the top Makefile is already too cluttered, this commit moves
all of extra gcc check stuff to a new file scripts/Makefile.extrawarn,
which is included from the top Makefile.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
CC: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>
diff --git a/Makefile b/Makefile
index 60ccbfe..8cc826a 100644
--- a/Makefile
+++ b/Makefile
@@ -105,10 +105,6 @@
   KBUILD_OUTPUT := $(O)
 endif
 
-ifeq ("$(origin W)", "command line")
-  export KBUILD_ENABLE_EXTRA_GCC_CHECKS := $(W)
-endif
-
 # That's our default target when none is given on the command line
 PHONY := _all
 _all:
@@ -731,6 +727,8 @@
 	KBUILD_CFLAGS += -DCC_HAVE_ASM_GOTO
 endif
 
+include $(srctree)/scripts/Makefile.extrawarn
+
 # Add user supplied CPPFLAGS, AFLAGS and CFLAGS as the last assignments
 KBUILD_CPPFLAGS += $(KCPPFLAGS)
 KBUILD_AFLAGS += $(KAFLAGS)