Bug #1450: Illumos should be buildable with GCC4
hal uses non-pointer NULL as g_strconcat sentinel
g_strconcat is a vararg function from glib, which takes a sentinel NULL. This should be pointer sized.
Unfortunately, in hal_dbus.c, we end up with one of our non-pointer NULL's, and GCC4 warns. We should cast this NULL to (void *), to be sure it ends up as (void *)0 or (void *)0L, and thus uintptr_t in size.
This basic bug is all over our codebase, but in ways which GCC cannot check. Future changes will allow GCC to notice, and expose them all (noisily).