[CMake] Allow C++ 11 standard.

CMakeLists.txt

@@ -41,8 +41,9 @@ set(PROJECT_DESCRIPTION
   )
 SET(PROJECT_USE_CMAKE_EXPORT TRUE)
 
-SET(CMAKE_C_STANDARD 99)
-SET(CMAKE_CXX_STANDARD 98)
+IF(NOT DEFINED CMAKE_CXX_STANDARD)
+  SET(CMAKE_CXX_STANDARD 98)
+ENDIF()
 
 # Do not support CMake older than 2.8.12
 CMAKE_POLICY(SET CMP0022 NEW)