[CMake] set CMAKE_OSX_SYSROOT

CMakeLists.txt

@@ -109,6 +109,7 @@ ELSE( CMAKE_SIZEOF_VOID_P EQUAL 8 )
   SET( LIB64  )
 ENDIF( CMAKE_SIZEOF_VOID_P EQUAL 8 )
 
+
 IF (APPLE AND (NOT PNACL))
     # Build 64 bit binaries compatible with OS X 10.7
     IF (NOT CMAKE_OSX_DEPLOYMENT_TARGET)
@@ -117,6 +118,14 @@ IF (APPLE AND (NOT PNACL))
     IF (NOT CMAKE_OSX_ARCHITECTURES)
         SET (CMAKE_OSX_ARCHITECTURES "x86_64" CACHE STRING "The processor architectures to build for" FORCE)
     ENDIF (NOT CMAKE_OSX_ARCHITECTURES)
+    IF (NOT CMAKE_OSX_SYSROOT)
+        EXECUTE_PROCESS(COMMAND "xcrun" "--show-sdk-path" OUTPUT_VARIABLE XCRUN_OSX_SYSROOT RESULT_VARIABLE XCRUN_OSX_SYSROOT_STATUS)
+        IF (XCRUN_OSX_SYSROOT_STATUS EQUAL 0)
+            SET (CMAKE_OSX_SYSROOT "${XCRUN_OSX_SYSROOT}" CACHE STRING "SDK Path" FORCE)
+        ENDIF (XCRUN_OSX_SYSROOT_STATUS EQUAL 0)
+        UNSET(XCRUN_OSX_SYSROOT)
+        UNSET(XCRUN_OSX_SYSROOT_STATUS)
+    ENDIF (NOT CMAKE_OSX_SYSROOT)
 
     # Improve the linking behavior of Mac libraries
     SET (CMAKE_INSTALL_NAME_DIR "@rpath")