SunPro: update flags used for CMake itself

Special flags should no longer be required when CMake is built with a
recent version of CMake.

CompileFlags.cmake

@@ -59,7 +59,8 @@ if(CMAKE_SYSTEM_PROCESSOR MATCHES "^parisc")
   endif()
 endif()
 
-if (CMAKE_CXX_COMPILER_ID STREQUAL SunPro)
+if (CMAKE_CXX_COMPILER_ID STREQUAL SunPro AND
+    NOT DEFINED CMAKE_CXX${CMAKE_CXX_STANDARD}_STANDARD_COMPILE_OPTION)
   if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 5.13)
     if (NOT CMAKE_CXX_STANDARD OR CMAKE_CXX_STANDARD EQUAL 98)
       set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++03")