apple-compiler-selection

• Builds targeting macOS no longer choose any SDK or pass an -isysroot flag to the compiler by default. Instead, compilers are expected to choose a default macOS SDK on their own. In order to use a compiler that does not do this, users must now specify -DCMAKE_OSX_SYSROOT=macosx when configuring their build.

• On macOS with Ninja Generators and Makefile Generators, when a compiler is found in /usr/bin, it is now used as-is and is no longer mapped to the corresponding compiler inside Xcode. The mapping was introduced by CMake 3.2 to allow build trees to continue to work with their original compiler even when xcode-select switches to a different Xcode installation. However, the compilers inside Xcode cannot be used without explicit -isysroot flags and are therefore not suitable for passing to arbitrary third-party build systems. Furthermore, the mapping behavior can override user-specified compiler paths. Therefore, this behavior has been reverted.