fs/proc: Add compiler check for -Wno-override-init to support gcc < 4.2

With gcc < 4.2 (e.g. 4.1.2):

      CC      fs/proc/task_mmu.o
    cc1: error: unrecognized command line option "-Wno-override-init"

To fix this, only enable the compiler option when it is actually
supported by the compiler.

Fixes: ca52953f ("fs/proc/task_mmu.c: suppress compilation warnings with W=1")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/Makefile

@@ -4,7 +4,7 @@
 
 obj-y   += proc.o
 
-CFLAGS_task_mmu.o	+= -Wno-override-init
+CFLAGS_task_mmu.o	+= $(call cc-option,-Wno-override-init,)
 proc-y			:= nommu.o task_nommu.o
 proc-$(CONFIG_MMU)	:= task_mmu.o