Don't use just GLIB_HAVE_ALLOCA_H to check if alloca is available,
that's just for the header. GLib may define alloca for us otherwise
too irrespective of GLIB_HAVE_ALLOCA_H.
Fixes compiler warning with mingw32:
gst/printf/vasnprintf.c:73:0: warning: "alloca" redefined
We will add support for our own printf modifiers, so we can
get nice debug log output on all operating systems irrespective
of the specific libc version used.
https://bugzilla.gnome.org/show_bug.cgi?id=613081