[MacPorts] #32949: drop dependency on gtk2, so librsvg can be made a dependency of gtk2

[MacPorts]

#32949: drop dependency on gtk2, so librsvg can be made a dependency of gtk2
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 Reporter:  cal@…             |       Owner:  pixilla@…           
     Type:  enhancement       |      Status:  new                 
 Priority:  Normal            |   Milestone:                      
Component:  ports             |     Version:  2.0.3               
 Keywords:                    |        Port:  librsvg             
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Comment(by cal@…):

 Since librsvg's documentation lacks information on why gtk is a dependency
 I took a look at the configure script and noticed the following:
  1. librsvg will not build the GTK+ theme engine, if gtk2 was not found
  2. librsvg will not build rsvg-view or rsvg-view-3, if neither gtk2 nor
 gtk3 were found
  3. librsvg will not generate HTML documentation with gtk-doc, if the gtk-
 doc port is not installed (which can be worked around by declaring a
 dependency on gtk-doc)

 So, dropping the dependency on gtk is not a no-op, but doable imho. We
 could move the support for gtk into a variant. I'll propose a patch.

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Ticket URL: <https://trac.macports.org/ticket/32949#comment:3>
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