clients & tests: Unify multiple definitions of x*alloc and related functions

Direct fail_on_null calls now produce output like:

    [weston-info] clients/weston-info.c:714: out of memory

xmalloc, et al produce output on failure like:

    [weston-info] out of memory (-1)

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/clients/cliptest.c b/clients/cliptest.c
index 07b0e2c..d0f4e3e 100644
--- a/clients/cliptest.c
+++ b/clients/cliptest.c
@@ -50,6 +50,7 @@
 #include <wayland-client.h>
 
 #include "src/vertex-clipping.h"
+#include "shared/xalloc.h"
 #include "window.h"
 
 typedef float GLfloat;