gmem.h: Use typeof() in g_steal_pointer() macro
gmem.h: Use typeof() in g_steal_pointer() macro

g_steal_pointer is both an inline function, returning gpointer, and a
macro that casts the return value to the type of its argument. The first
version of the macro uses '0 ? (*(pp)) : (g_steal_pointer) (pp)' to cast
the return value to the type of *pp, but this fails to yield warnings
about incompatible pointer types with current gcc. Apparently the
ternary operator is optimized away before the type of the expression is
determined.

The typeof() (or __typeof__()) operator allows an explicit cast.

https://bugzilla.gnome.org/show_bug.cgi?id=742456

https://bugzilla.gnome.org/show_bug.cgi?id=796341

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib