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

As discussed in chat, -Werror should not be enabled by default.
Instead it should be added in our jenkins build scripts.

This reverts commit 7d1f53143e8eaeb25bae8979fb80dac57ddbf827.

Change-Id: Ied6e21b72b24fcb623fd945ed9560b742a8eedf5
diff --git a/configure.ac b/configure.ac
index b76545e..535d1c3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -26,8 +26,8 @@
 
 AC_CONFIG_MACRO_DIR([m4])
 
-CFLAGS+=" -Wall -Werror"
-CPPFLAGS+=" -Wall -Werror"
+CFLAGS+=" -Wall"
+CPPFLAGS+=" -Wall"
 
 # The following test is taken from WebKit's webkit.m4
 saved_CFLAGS="$CFLAGS"