with-display-message: Workaround for bug#63253

with-display-message: Workaround for bug#63253

Running arbitrary ELisp code from an atimer is still dangerous,
at least because the regexp engine is not-reentrant, so let's patch up
the case we bumped into.  There are probably many other such holes :-(

* src/alloc.c (garbage_collection_inhibited): Make it non-static.

* src/xdisp.c (garbage_collection_inhibited): Declare it.
(set_message, clear_message): Use it as a proxy for "we're in
a dangerous context like within `probably_quit`".