commit e6fb95871cd3557e5882a6f969c11bc6a3cbe781
Author: Florian Weimer <fweimer@redhat.com>
Date:   Mon Sep 8 13:49:36 2014 +0200

    Turn on -Werror=implicit-function-declaration
    
    GCC 4.4, the minimum compiler version, supports this option.  Unlike
    other warnings, -Wimplicit-function-declaration warnings should be
    independent of compiler versions, so this change should not cause
    compiler-specific build failures.

Index: b/Makeconfig
===================================================================
--- a/Makeconfig
+++ b/Makeconfig
@@ -643,7 +643,7 @@ ifeq ($(all-warnings),yes)
 else
 +gccwarn := -Wall -Wwrite-strings -Winline
 endif
-+gccwarn-c = -Wstrict-prototypes
++gccwarn-c = -Wstrict-prototypes -Werror=implicit-function-declaration
 
 # We do not depend on the address of constants in different files to be
 # actually different, so allow the compiler to merge them all.