OS X: Do not default to non-existent deployment target SDK

Since commit 95f78e08 (OS X: Search for SDK based on deployment target,
2013-08-02) we select the default OS X SDK path to match the deployment
target.  Fix this behavior in the case that the matching SDK does not
exist and fall back to the SDK for the current host OS X version.

Modules/Platform/Darwin.cmake

@@ -134,11 +134,23 @@ elseif("${CMAKE_GENERATOR}" MATCHES Xcode
     set(_CMAKE_OSX_SDKS_VER_SUFFIX_10.3 ".9")
     if(CMAKE_OSX_DEPLOYMENT_TARGET)
       set(_CMAKE_OSX_SDKS_VER ${CMAKE_OSX_DEPLOYMENT_TARGET}${_CMAKE_OSX_SDKS_VER_SUFFIX_${CMAKE_OSX_DEPLOYMENT_TARGET}})
+      set(_CMAKE_OSX_SYSROOT_CHECK "${_CMAKE_OSX_SDKS_DIR}/MacOSX${_CMAKE_OSX_SDKS_VER}.sdk")
+      if(IS_DIRECTORY "${_CMAKE_OSX_SYSROOT_CHECK}")
+        set(_CMAKE_OSX_SYSROOT_DEFAULT "${_CMAKE_OSX_SYSROOT_CHECK}")
+      else()
+        set(_CMAKE_OSX_SDKS_VER ${_CURRENT_OSX_VERSION}${_CMAKE_OSX_SDKS_VER_SUFFIX_${_CURRENT_OSX_VERSION}})
+        set(_CMAKE_OSX_SYSROOT_DEFAULT "${_CMAKE_OSX_SDKS_DIR}/MacOSX${_CMAKE_OSX_SDKS_VER}.sdk")
+        message(WARNING
+          "CMAKE_OSX_DEPLOYMENT_TARGET is '${CMAKE_OSX_DEPLOYMENT_TARGET}' "
+          "but the matching SDK does not exist at:\n \"${_CMAKE_OSX_SYSROOT_CHECK}\"\n"
+          "Instead using SDK:\n \"${_CMAKE_OSX_SYSROOT_DEFAULT}\"\n"
+          "matching the host OS X version."
+          )
+      endif()
     else()
       set(_CMAKE_OSX_SDKS_VER ${_CURRENT_OSX_VERSION}${_CMAKE_OSX_SDKS_VER_SUFFIX_${_CURRENT_OSX_VERSION}})
+      set(_CMAKE_OSX_SYSROOT_DEFAULT "${_CMAKE_OSX_SDKS_DIR}/MacOSX${_CMAKE_OSX_SDKS_VER}.sdk")
     endif()
-    set(_CMAKE_OSX_SYSROOT_DEFAULT
-      "${_CMAKE_OSX_SDKS_DIR}/MacOSX${_CMAKE_OSX_SDKS_VER}.sdk")
   else()
     # Assume developer files are in root (such as Xcode 4.5 command-line tools).
     set(_CMAKE_OSX_SYSROOT_DEFAULT "")