commit 48bb14bdbbeb09cb3cd950d7346688958f1bce1a
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Thu Aug 20 20:50:05 2015 +0000

    Don't use -Wno-error=undef.
    
    This patch removes the use of -Wno-error=undef, so that -Wundef
    warnings become errors.
    
    Tested for x86_64, x86, mips64 (all three ABIs) and arm.
    
            * Makeconfig [$(enable-werror) = yes] (+gccwarn): Do not add
            -Wno-error=undef.

diff --git a/Makeconfig b/Makeconfig
index b1bf7f24b6154e3d..3c0d4abc5f5f30df 100644
--- a/Makeconfig
+++ b/Makeconfig
@@ -645,7 +645,7 @@ else
 endif
 +gccwarn += -Wundef
 ifeq ($(enable-werror),yes)
-+gccwarn += -Werror -Wno-error=undef
++gccwarn += -Werror
 endif
 +gccwarn-c = -Wstrict-prototypes