[CMake] Manually set the OS X SDK and deployment target.

...so that CMake doesn't try to "helpfully" pass -mmacosx-version-min=
when linking libraries for other platforms. (This only came up in
certain build configurations.) Fix by @cwakamo.

rdar://problem/25271364

Author: jrose-apple

CMakeLists.txt

@@ -699,6 +699,13 @@ if(SWIFT_PARALLEL_LINK_JOBS)
   endif()
 endif()
 
+# Set the CMAKE_OSX_* variables in a way that minimizes conflicts.
+if("${CMAKE_SYSTEM_NAME}" STREQUAL "Darwin")
+  set(CMAKE_OSX_SYSROOT "${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_PATH}")
+  set(CMAKE_OSX_ARCHITECTURES "")
+  set(CMAKE_OSX_DEPLOYMENT_TARGET "")
+endif()
+
 message(STATUS "Building host Swift tools for ${SWIFT_HOST_VARIANT_SDK} ${SWIFT_HOST_VARIANT_ARCH}")
 message(STATUS "  Build type: ${CMAKE_BUILD_TYPE}")
 message(STATUS "  Assertions: ${LLVM_ENABLE_ASSERTIONS}")