MacOS Unified Application does not run on El Capitan
The MacOS Unified Application, currently built in a High Sierra (10.13) environment, does not run on El Capitan (10.11). The reason is that the version of libffi packaged within the unified app (libffi.6.dylib) references a function in libSystem (mkostemp) that does not exist in the El Capitan version of that library. Since libSystem is not packaged within the unified app (and shouldn't be), libffi fails to load on El Capitan, and Chirp dies on startup.
I don't have a Sierra (10.12) installation around to test on, so I don't know if this issue also arises with that version.
It's not clear to me what the solution is, if there is a feasible one other than building the unified app on El Capitan instead / also. The libffi library appears to be part of MacOS, so it may be possible to package less within the app. I see several other apps / frameworks on my machine that include libffi, and it's interesting that they all also include pango, so I wonder if perhaps more is being brought in with pango than necessary.
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