anything less than clang 3.5 is unsupported

cmake/Modules/ClhepVariables.cmake

@@ -88,8 +88,8 @@ endmacro( clhep_autoconf_variables )
 
 macro( _clhep_verify_cxx11 )
   if( ${CMAKE_CXX_COMPILER_ID} STREQUAL "Clang" )
-    if( CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.3 )
-      message( FATAL_ERROR "c++11 extension is not available for ${CMAKE_CXX_COMPILER_ID} ${CMAKE_CXX_COMPILER_VERSION}")
+    if( CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5 )
+      message( FATAL_ERROR "CLHEP now requires c++11 support with clang 3.5 or later")
     else()
       set( HAVE_STDCXX true )
     endif()