Re: Crash in 2.25.4

[Jean Abou Samra]

Gwyn, is it convenient for you to test patches? I think I see what this is 
caused by, but I cannot reproduce the bug in a local build (knowing what exact 
compiler flags are used in Fedora's builds might help).

I'd like to know if this fixes it:

```
diff --git a/lily/all-font-metrics-scheme.cc b/lily/all-font-metrics-scheme.cc
index 9a36407993..77fabc666b 100644
--- a/lily/all-font-metrics-scheme.cc
+++ b/lily/all-font-metrics-scheme.cc
@@ -24,7 +24,7 @@
 
 All_font_metrics *all_fonts_global = nullptr;
 // GC-protects the all_fonts_global object.
-SCM all_fonts_global_scm = SCM_UNDEFINED;
+volatile SCM all_fonts_global_scm = SCM_UNDEFINED;
 
 LY_DEFINE (ly_reset_all_fonts, "ly:reset-all-fonts", 0, 0, 0, (),
            R"(
```

Thank you.

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