[Ohrrpgce] Warning when running a 32-bit Mac .app

[Ralph Versteegen]

Someone mentioned that OSX showed a warning when they ran the OHR. I
discovered (Wikipedia):

"In macOS High Sierra (10.13.4), a warning is issued the first time a
32-bit app is opened that 32-bit apps will not be supported in future
updates. In macOS Mojave, this alert appears once every 30 days when
launching the app, as the next version of macOS will not support
them."

It looks like this: https://support.apple.com/en-us/HT208436

New OSX versions are generally released in September.
OS 10.5 (2007) was the first release that had 64-bit support, though
apparently it's more complete support in 10.6 (2009). We currently
target 10.4, but I was thinking of switching to 10.5 or 10.6 anyway.

We can already produce 64 bit binaries (that was happening
accidentally at one point) but they need to be combined with 64-bit
copies of SDL and SDL_mixer. I think the frameworks downloaded from
libsdl.org actually include both x86 and x86_64 but I deleted the
x86_64 libraries (and also the huge OpenGL header files).