GraphicsMagick: disable SIGSEGV handler in library; add debug variant

[mascguy (Christopher Nielsen)]

Presently, we're trying to diagnose a crash occurring for darktable, which appears to be originating from within this port:

issue:64252

There are two barriers, though:

• The GraphicsMagick library installs a SIGSEGV handler, preventing the app from catching/handling the condition;
• No +debug variant, which would assist with diagnostics.

Relative to the signal handler, we should consider patching the code to disable that, for the library. Along with engaging with upstream, for a longer-term fix.

As for the debug variant, that should be reasonably straightforward, and would be beneficial.

[mascguy (Christopher Nielsen)]

Too much on my plate, I'll let Ryan tackle this

[mascguy (Christopher Nielsen)]

issue:64252 has been resolved, so this is no longer needed.

If there's user demand, we can always reopen.

[jmroot (Joshua Root)]

There is still the valid issue raised in the other ticket that GraphicsMagick installs its own SIGSEGV handler when used as a library. Up to you if you want to open a ticket for that, here or upstream.

[mascguy (Christopher Nielsen)]

Let's expand the scope of this ticket to include the lib signal handler, per @jmroot in comment:3