...by pretending to be OS X 10.9. This might remove the need for other
workarounds as well, but I'm not going to go track those down right now.
Swift SVN r13886
The Swift stdlib build uses CMAKE_FORCE_C_COMPILER to cross-compile.
When the compiler is forced cmake does not set the compiler version
variables that llvm's build started checking in llvm commit bf2609e26.
Swift SVN r12311