OS X: Search system SDKs for frameworks

In Modules/Platform/Darwin.cmake set CMAKE_SYSTEM_FRAMEWORK_PATH to
include framework directories from inside the system SDK corresponding
to CMAKE_OSX_SYSROOT.
Suggested-by: Sean McBride <sean@rogue-research.com>

Modules/Platform/Darwin.cmake

@@ -294,6 +294,15 @@ endif()
 # set up the default search directories for frameworks
 set(CMAKE_SYSTEM_FRAMEWORK_PATH
   ~/Library/Frameworks
+  )
+if(_CMAKE_OSX_SYSROOT_PATH)
+  list(APPEND CMAKE_SYSTEM_FRAMEWORK_PATH
+    ${_CMAKE_OSX_SYSROOT_PATH}/Library/Frameworks
+    ${_CMAKE_OSX_SYSROOT_PATH}/Network/Library/Frameworks
+    ${_CMAKE_OSX_SYSROOT_PATH}/System/Library/Frameworks
+    )
+endif()
+list(APPEND CMAKE_SYSTEM_FRAMEWORK_PATH
   /Library/Frameworks
   /Network/Library/Frameworks
   /System/Library/Frameworks)