Merge pull request #1336 from gangliao/macosx

set macosx deployment version

cmake/system.cmake

@@ -28,6 +28,11 @@ ELSE(WIN32)
         STRING(REGEX MATCH "[0-9]+.[0-9]+" VERSION "${MACOSX_VERSION}")
         SET(MACOS_VERSION ${VERSION})
         SET(HOST_SYSTEM "macosx")
+        IF(NOT DEFINED ENV{MACOSX_DEPLOYMENT_TARGET})
+            # Set cache variable - end user may change this during ccmake or cmake-gui configure.
+            SET(CMAKE_OSX_DEPLOYMENT_TARGET ${MACOS_VERSION} CACHE STRING
+                "Minimum OS X version to target for deployment (at runtime); newer APIs weak linked. Set to empty string for default value.")
+        ENDIF()
     ELSE(APPLE)
 
         IF(EXISTS "/etc/issue")