Ok, I'm giving this a spin now, following Edmundito's instructions.
The step "make the libraries" fails. If I then do a checkout to v.3.4.0.13 and make again, it again fails. The error I'm getting is 'QuickTime/QuickTime.h' file not found.
Any advice on how to proceed from here please?
(edit) wait, I thought this was El Capitan but it turns out to be High Sierra. SDK 10.11 did the trick. The make process throws a lot of warnings but nothing stands out as problematic.
(edit edit) nope. I cheeck out to master again; replace the file.c file; remove the patch line from the Makefile and make again. This doesn't work. The two errors are cannot find interface declaration for NSQuickDrawView, superclass of llegroView; and unknown type name CGDirectPaletteRef. Maybe I should just try building 3.4.0 instead of the latest AGS version?
(edit edit edit) ignoring the above error, I open Xcode. First I get two errors to "change to recommended settings", which I allow. Now I get eight 'multiple build commands' warnings which don't seem important, and an allegro.h file not found (in draw.h)...