kbuild: check for wrong use of CFLAGS

External modules have in a few cases modifed gcc option
by modifying CFLAGS. This has never been documented and
was a bad practice.
With the check to use KBUILD_CFLAGS it will no longer work
so we better error out and tell what was wrong as a service
to the external module users.

This check can be overruled if
KBUILD_NOPEDANTIC is set to something.
Addid this possibility may allow older external
module to build without any code modifications but potentially
only loosing some un-important gcc options.
Signed-off-by: NSam Ravnborg <sam@ravnborg.org>

scripts/Makefile.build

@@ -28,10 +28,20 @@ EXTRA_LDFLAGS  :=
 
 include scripts/Kbuild.include
 
+# For backward compatibility check that these variables does not change
+save-cflags := $(CFLAGS)
+
 # The filename Kbuild has precedence over Makefile
 kbuild-dir := $(if $(filter /%,$(src)),$(src),$(srctree)/$(src))
-include $(if $(wildcard $(kbuild-dir)/Kbuild), $(kbuild-dir)/Kbuild, $(kbuild-dir)/Makefile)
-
+kbuild-file := $(if $(wildcard $(kbuild-dir)/Kbuild),$(kbuild-dir)/Kbuild,$(kbuild-dir)/Makefile)
+include $(kbuild-file)
+
+# If the save-* variables changed error out
+ifeq ($(KBUILD_NOPEDANTIC),)
+        ifneq ("$(save-cflags)","$(CFLAGS)")
+                $(error CFLAGS was changed in "$(kbuild-file)". Fix it to use EXTRA_CFLAGS)
+        endif
+endif
 include scripts/Makefile.lib
 
 ifdef host-progs