autogen.sh: Prepare CFLAGS before ./configure

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

autogen.sh

@@ -2,8 +2,12 @@
 
 autoreconf -v -f --install "$(dirname "$0")" || exit 1
 
+CFLAGS="-O2"
+CFLAGS="$CFLAGS -Wall"
+CFLAGS="$CFLAGS -Werror=implicit-function-declaration"
+
 "$(dirname "$0")"/configure \
-	CFLAGS="-O2 -Wall -Werror=implicit-function-declaration" \
+	CFLAGS="$CFLAGS" \
 	--enable-lastlog \
 	--enable-man \
 	--enable-maintainer-mode \