[PATCH v3] Makefile: enable -Werror=implicit-int and -Werror=strict-prototypes by default

[Sergei Trofimovich <slyfox@gentoo.org>]

The common error fount in forward-ported/backported patches is missing headers.
One recent example (files and function names are mangled):

    void foo(){}
    EXPORT_SYMBOL(foo);

gave only warning

    foo.c:12345678:5: warning: function declaration isn't a prototype [-Wstrict-prototypes]
    void foo(){}
         ^

    foo.c:12345679:5: warning: data definition has no type or storage class [enabled by default]
    EXPORT_SYMBOL(foo);
    foo.c:12345679:5: warning: type defaults to 'int' in declaration of 'EXORT_SYMBOL' [-Werror=implicit-int]

Now it's a fatal error. Tested on x86_64 allyesconfig.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Cc: Michal Marek <mmarek@suse.cz>
Cc: linux-kbuild@vger.kernel.org
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
---
 Makefile | 6 +++++++
 1 file changed, 6 insertions(+)

Change since v2:
 - moved CFLAGS checks lower to catch $(CROSS) case
   as suggested by Geert
 - added comments as other warning options do

Change since v1:
 - use 'cc-option' to respect old gccs
 - fix typos. Thanks to Oleg Verych

diff --git a/Makefile b/Makefile
index a5a55f4..f5f36e1 100644
--- a/Makefile
+++ b/Makefile
@@ -659,6 +660,12 @@ KBUILD_CFLAGS	+= $(call cc-option,-fno-strict-overflow)
 # conserve stack if available
 KBUILD_CFLAGS   += $(call cc-option,-fconserve-stack)
 
+# disallog errors like 'EXPORT_GPL(foo);' with missing header
+KBUILD_CFLAGS   += $(call cc-option,-Werror=implicit-int)
+
+# require functions to have argumens in prototypes, not empty 'int foo()'
+KBUILD_CFLAGS   += $(call cc-option,-Werror=strict-prototypes)
+
 # use the deterministic mode of AR if available
 KBUILD_ARFLAGS := $(call ar-option,D)
 
-- 
1.8.3.2