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Is it time to pull the plug on the older versions of MacOS?
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So I'm inclined to change the support policy to 5 years, effective on the fifth anniversary of Mountain Lion's release on 16 February; meaning that after that gtk-osx won't actively support building on systems older than Mountain Lion; on 10 June 2018 the minimum supported system will be Mavericks, and so on.
The immediate impact will be removing all of the ppc-related and pre-10.8 special case code from jhbuildrc-gtk-osx. Over time I'd also weed out patches and modules which enable building on now unsupported versions of OS X.
I'm really late to this thread, but yes, I wholeheartedly support this as well.
In fact while working on my build stuff [1] late last year, I discovered that you can actually just use the normal GNOME JHbuild modulesets on 10.12, and it should work on 10.9 and later. I got most of my patches accepted upstream by now. I don't know about 10.8, I just stripped out all the code that dealt with 10.8 and earlier because GTK would need to be patched to handle 10.8; but it may be possible to support 10.8 with a minimum of fuss.
I have been meaning to start a discussion about merging [1] into upstream JHbuild as you suggested, but didn't have the time. However, I guess unless you could use the upstream modules to build Gnucash, you'd still need to maintain gtk-osx...
Regards,
Philip C