[Lazarus] Issue 21129, hang on glib2 2.31

[Ludo Brands]

Hi,

the issue is read-only now, so I comment here. 

The patch commited leaves USE_GTK_MAIN_OLD_ITERATION as before, using
gdk_threads_init. It would be useful to note somewhere in the sources
(gtk2defines.inc?) that USE_GTK_MAIN_OLD_ITERATION causes hangs when used
with glib2 2.31. USE_GTK_MAIN_OLD_ITERATION is only used with non-Unix
systems but the day 2.31 or + is ported to windows we probably all forgot.

Ludo