autogen.sh: CFLAGS: Add -Werror=implicit-function-declaration

This is not just a style issue.  This should be a hard error, and never
compile.  ISO C89 already had this feature as deprecated.  ISO C99
removed this deprecated feature, for good reasons.  If we compile
ignoring this warning, shadow is not going to behave well.

Cc: Sam James <sam@gentoo.org>
Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

autogen.sh

@@ -3,7 +3,7 @@
 autoreconf -v -f --install "$(dirname "$0")" || exit 1
 
 "$(dirname "$0")"/configure \
-	CFLAGS="-O2 -Wall" \
+	CFLAGS="-O2 -Wall -Werror=implicit-function-declaration" \
 	--enable-lastlog \
 	--enable-man \
 	--enable-maintainer-mode \