After updating to macOS Mojave 10.14.1, after fixing fpc and reinstalling the XCode Command Line Tools and can now again compile USDX without problems.
However, when I start the game, I only get a black screen. The game is running, I can hear the background music, can (blindly) navigate to exit the program, so this is merely a graphics/display problem.
As discussed in #389, this is most likely this Bug: https://bugzilla.libsdl.org/show_bug.cgi?id=4272
Thanks for the quick reply, @s09bQ5. I missed the discussion in #389.
Current workaround that works for me: edit config.ini and set FullScreen=Off, then start the game, double-click on the window to resize to full window, then hit Alt+Enter to go to borderless.
Good news, this issue is somewhat gone. In the end, USDX behaves as expected. However, the loading screen is still a black screen (that shows the colored ball for loading), which then turns into the start screen of USDX. So while it is not the expected behavior, it is now only a minor issue.
What has changed? I have updated and upgraded my brew packages and assume that SDL has been updated as well. However, I don't know how I could verify if that is the case.