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

Change-Id: I5033387e4a4206f7d34d1db3c6ede9255d199941
diff --git a/configure.ac b/configure.ac
index ff78035..535d1c3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -26,6 +26,9 @@
 
 AC_CONFIG_MACRO_DIR([m4])
 
+CFLAGS+=" -Wall"
+CPPFLAGS+=" -Wall"
+
 # The following test is taken from WebKit's webkit.m4
 saved_CFLAGS="$CFLAGS"
 CFLAGS="$CFLAGS -fvisibility=hidden "