kbuild: Introdude KBUILD_NOCMDDEP

When tossing around with different gcc compilers there is no way to tell kbuild
to ignore the new name of the compiler. The new option KBUILD_NOCMDDEP tell
kbuild not to check the commandline for changes.
This should be used with care because the resulting kernel may become inconsistent
if one part is build with 2.96, and another part build with 3.3.4.
So use only when you know what you are doing.

Syntax:
make KBUILD_NOCMDDEP=1

Original request for this feature came from hpa.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

scripts/Makefile.lib

@@ -183,8 +183,12 @@ cmd_gzip = gzip -f -9 < $< > $@
 # Generic stuff
 # ===========================================================================
 
+ifneq ($(KBUILD_NOCMDDEP),1)
 # Check if both arguments has same arguments. Result in empty string if equal
-arg-check := $(strip $(filter-out $(1), $(2)) $(filter-out $(2), $(1)) )
+# User may override this check using make KBUILD_NOCMDDEP=1
+_arg-check = $(strip $(filter-out $(1), $(2)) $(filter-out $(2), $(1)) )
+
+endif
 
 # echo command. Short version is $(quiet) equals quiet, otherwise full command
 echo-cmd = $(if $($(quiet)cmd_$(1)), \