[MacPorts] #65630: GraphicsMagick: add debug variant

[MacPorts]

#65630: GraphicsMagick: add debug variant
-------------------------+----------------------------
 Reporter:  mascguy      |      Owner:  mascguy
     Type:  enhancement  |     Status:  assigned
 Priority:  Normal       |  Milestone:
Component:  ports        |    Version:  2.7.2
 Keywords:  debug        |       Port:  GraphicsMagick
-------------------------+----------------------------
 Presently, we're trying to diagnose a crash occurring for `darktable`,
 which appears to be originating from within this port:

 issue:64252

 Add support for `+debug`, to help with such cases.

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Ticket URL: <https://trac.macports.org/ticket/65630>
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