diff --git a/configure.ac b/configure.ac
index e7ea534..13f5dc2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -14,6 +14,7 @@ AM_MAINTAINER_MODE
 dnl Enable compile warnings (only supporting GCC)
 if test "x$GCC" == xyes; then
     WARN_CFLAGS="-Wall \
+    		 -std=gnu99 \
                  -Wstrict-prototypes \
                  -Wnested-externs \
                  -Werror=missing-prototypes \
diff --git a/src/x-authority.h b/src/x-authority.h
index 748e3f1..116cce0 100644
--- a/src/x-authority.h
+++ b/src/x-authority.h
@@ -68,7 +68,7 @@ void x_authority_set_address (XAuthority *auth, const guint8 *address, gsize add
 
 const guint8 *x_authority_get_address (XAuthority *auth);
 
-gsize x_authority_get_address_length (XAuthority *auth);
+const gsize x_authority_get_address_length (XAuthority *auth);
 
 void x_authority_set_number (XAuthority *auth, const gchar *number);