Index: cmake/Modules/CompilerRTDarwinUtils.cmake
===================================================================
--- /dev/null
+++ cmake/Modules/CompilerRTDarwinUtils.cmake
@@ -0,0 +1,78 @@
+# On OS X SDKs can be installed anywhere on the base system and xcode-select can
+# set the default Xcode to use. This function finds the SDKs that are present in
+# the current Xcode.
+function(find_darwin_sdk_dir var sdk_name)
+  # Let's first try the internal SDK, otherwise use the public SDK.
+  execute_process(
+    COMMAND xcodebuild -version -sdk ${sdk_name}.internal Path
+    OUTPUT_VARIABLE var_internal
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+    ERROR_FILE /dev/null
+  )
+  if("" STREQUAL "${var_internal}")
+    execute_process(
+      COMMAND xcodebuild -version -sdk ${sdk_name} Path
+      OUTPUT_VARIABLE var_internal
+      OUTPUT_STRIP_TRAILING_WHITESPACE
+      ERROR_FILE /dev/null
+    )
+  endif()
+  set(${var} ${var_internal} PARENT_SCOPE)
+endfunction()
+
+# There isn't a clear mapping of what architectures are supported with a given
+# target platform, but ld's version output does list the architectures it can
+# link for.
+function(darwin_get_toolchain_supported_archs output_var)
+  execute_process(
+    COMMAND ld -v
+    ERROR_VARIABLE LINKER_VERSION)
+
+  string(REGEX MATCH "configured to support archs: (.*)\nLTO support.*"
+         ARCHES_MATCHED "${LINKER_VERSION}")
+  if(ARCHES_MATCHED)
+    set(ARCHES "${CMAKE_MATCH_1}")
+    message("Matched ARCHES: ${ARCHES}")
+    string(REPLACE " " ";" ARCHES ${ARCHES})
+  else()
+    # If auto-detecting fails, fall back to a default set
+    message(WARNING "Detecting supported architectures from 'ld -v' failed. Returning default set.")
+    set(ARCHES "i386;x86_64;armv7;armv7s;arm64")
+  endif()
+  
+  set(${output_var} ${ARCHES} PARENT_SCOPE)
+endfunction()
+
+# This function takes an OS and a list of architectures and identifies the
+# subset of the architectures list that the installed toolchain can target.
+function(darwin_test_archs os valid_archs)
+  set(archs ${ARGN})
+  message(STATUS "Finding valid architectures for ${os}...")
+  set(SIMPLE_CPP ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/src.cpp)
+  file(WRITE ${SIMPLE_CPP} "#include <iostream>\nint main() { return 0; }\n")
+
+  set(os_linker_flags)
+  foreach(flag ${DARWIN_${os}_LINKFLAGS})
+    set(os_linker_flags "${os_linker_flags} ${flag}")
+  endforeach()
+
+  # The simple program will build for x86_64h on the simulator because it is 
+  # compatible with x86_64 libraries (mostly).
+  if(${os} STREQUAL "iossim")
+    list(REMOVE_ITEM archs "x86_64h")
+  endif()
+
+  set(WORKING_ARCHES)
+  foreach(arch ${archs})
+    
+    set(arch_linker_flags "-arch ${arch} ${os_linker_flags}")
+    try_compile(CAN_TARGET_${os}_${arch} ${CMAKE_BINARY_DIR} ${SIMPLE_CPP}
+                COMPILE_DEFINITIONS "-v -arch ${arch}" ${DARWIN_${os}_CFLAGS}
+                CMAKE_FLAGS "-DCMAKE_EXE_LINKER_FLAGS=${arch_linker_flags}"
+                OUTPUT_VARIABLE TEST_OUTPUT)
+    if(${CAN_TARGET_${os}_${arch}})
+      list(APPEND WORKING_ARCHES ${arch})
+    endif()
+  endforeach()
+  set(${valid_archs} ${WORKING_ARCHES} PARENT_SCOPE)
+endfunction()
\ No newline at end of file