require at least gcc 4.8

cmake/Modules/ClhepVariables.cmake

@@ -119,10 +119,8 @@ macro( _clhep_verify_cxx11 )
       set( HAVE_STDCXX true )
     endif()
   elseif(CMAKE_COMPILER_IS_GNUCXX)
-    if( CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.7 )
-      message( FATAL_ERROR "c++11 extension is not available for ${CMAKE_CXX_COMPILER}")
-    elseif( CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.8 )
-      message( FATAL_ERROR "CLHEP now requires gcc 4.8 or later")
+    if( CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.8 )
+      message( FATAL_ERROR "CLHEP now requires c++11 support with gcc 4.8 or later")
     else()
       set( HAVE_STDCXX true )
     endif()