glib2: implement new segregated port scheme, with separate x11/quartz subports, to support multiple versions installed side-by-side

[mascguy (Christopher Nielsen)]

Given the challenges updating glib2 - there are a tremendous number of dependent ports, requiring a corresponding amount of integration testing, headaches, etc - it's time to consider a segregated scheme, in the spirit of boost.

The goal is to keep our two legacy ports - glib2 and glib2-devel - as-is. This avoids any disruptions, breakages, etc.

But for those ports that require the latest upstream version, they'll be able to utilize the new scheme. And since X11 and Quartz will be supported by separate non-conflicting subports, that will further reduce headaches.

This work dovetails with eventual segregation of GTK-related ports, with a similar X11/Quartz subport strategy; tracked by long-standing ticket: issue:27990

[mascguy (Christopher Nielsen)]

Update for interested parties:

• I'm going to tackle this ASAP, via our new port glib2-upstream.
• Once that's done, I'll review and coordinate with Ryan relative to folding this functionality into glib2-devel. (And eventually glib2.)

In terms of overall strategy:

• Given the number of dependent ports, the overarching goal is to make the transition as seamless as possible.
• To that end, two new segregated subports will added: glib2-upstream-x11 and glib2-upstream-quartz.
• The base port will remain as-is for backward compatibility, installing everything where it goes today. That way dependents can gradually be migrated to the new scheme.