GMemVTable broken

[Alexander Larsson <alexl redhat com>]

So, I just tried to use the memory profiler in glib, and I crashes
really early because the gobject constructor (gobject_init_ctor) calls
g_malloc before main() is reached.

This means g_mem_set_vtable() is impossible to use. I don't necessarily
think this is all that bad. Honestly we should never have made it a
vtable (one extra vfunc per malloc...) and instead do memory profiling
etc the "normal" way, i.e. LD_PRELOAD something with malloc
interceptors. Maybe we should just remove all this code and keep
g_mem_set_vtable as a dummy function that prints a deprecation warning?