RTTI is a C++ feature that enables fancy type magic (particularly dynamic_cast).
In 0 A.D., we do not use it (per CCs), as it is generally an indicator of bad/convoluted/weird code and as it is quite slow (see e.g. this where it's 100x slower than alternatives, I expect it's not as bad in general, but it's definitely slow).
D2768 inherits a class from Spidermonkey, and Spidermonkey is compiled without RTTI (up to at least SM78). This doesn't work if we compile with RTTI.
Therefore, we must de-activate RTTI on all platforms. This was already done on Windows, but on on mac/linux.
AtlasUI is left unaffected (presumably wxWidgets relies on it).
See comments on D1781 for more info on dynamic_cast.