Fix MacOS runtime error in Gloss apps

[turion]

I'm hoping to fix #280 .

• [x] Test whether this is in fact the relevant change (@reubenharry can you help maybe?)
• [ ] Document in the rhine-gloss readme
• [ ] Port to other gloss example
• [ ] Open analogous tickets for sonnendemo and eolc
• [ ] Maybe create a smoke test for the gloss apps that can run on CI also on mac?

[turion]

But note that not having -threaded may lead to worse performance on Macs, as then all GHC threads are blocked by OpenGL FFI calls.