ENH: protect the export files against inclusion with cmake 2.4

Alex

Source/cmExportFileGenerator.cxx

@@ -79,6 +79,12 @@ bool cmExportFileGenerator::GenerateImportFile()
     }
   std::ostream& os = *foutPtr;
 
+  // Protect that file against use with older CMake versions.
+  os << "# Generated by CMake " << cmVersion::GetCMakeVersion() << "\n\n";
+  os << "IF(\"${CMAKE_MAJOR_VERSION}.${CMAKE_MINOR_VERSION}\" LESS 2.5)\n"
+     << "   MESSAGE(FATAL_ERROR \"CMake >= 2.6.0 required\")\n"
+     << "ENDIF(\"${CMAKE_MAJOR_VERSION}.${CMAKE_MINOR_VERSION}\" LESS 2.5)\n";
+
   // Isolate the file policy level.
   // We use 2.6 here instead of the current version because newer
   // versions of CMake should be able to export files imported by 2.6