configure.ac: Use -Werror in C(PP)FLAGS

Change-Id: Ic674bf28d781b9806dc023777e1fea4ee1a0257a
diff --git a/configure.ac b/configure.ac
index 535d1c3..b76545e 100644
--- a/configure.ac
+++ b/configure.ac
@@ -26,8 +26,8 @@
 
 AC_CONFIG_MACRO_DIR([m4])
 
-CFLAGS+=" -Wall"
-CPPFLAGS+=" -Wall"
+CFLAGS+=" -Wall -Werror"
+CPPFLAGS+=" -Wall -Werror"
 
 # The following test is taken from WebKit's webkit.m4
 saved_CFLAGS="$CFLAGS"