This adds a dependency on a version of CMake with support for Objective-C,
removing a number of hacks to make CMake work and introduces a few
modernisations:
- Options use CMake's `option` mechanism.
- A lot of things now use generator expressions, which should improve the
things like the multi-config Ninja generator.
- We no longer depend on hacks to build in CI.
At the same time, a few other things are simplified:
- There is no longer support for building without C++ support and we depend on
a C++ standard library, not just a runtime. This makes it possible to use
more C++ features inside the runtime.
- We no longer support clang-cl on Windows and instead require clang.exe.
- Support for the Boehm GC has been removed.
David Chisnall