diff --git a/Source/cmMakefile.cxx b/Source/cmMakefile.cxx
index c26d5ab12..b0dfb41fa 100644
--- a/Source/cmMakefile.cxx
+++ b/Source/cmMakefile.cxx
@@ -921,7 +921,20 @@ const char* cmMakefile::GetDefinition(const char* name) const
       }
     else 
       {
-      vv->VariableAccessed(name, cmVariableWatch::UNKNOWN_VARIABLE_READ_ACCESS);
+      // are unknown access allowed
+      DefinitionMap::const_iterator pos2 = 
+        m_Definitions.find("CMAKE_ALLOW_UNKNOWN_VARIABLE_READ_ACCESS");
+      if (pos2 != m_Definitions.end() && 
+          cmSystemTools::IsOn((*pos2).second.c_str())) 
+        {
+        vv->VariableAccessed(name, 
+                             cmVariableWatch::ALLOWED_UNKNOWN_VARIABLE_READ_ACCESS);
+        }
+      else
+        {
+        vv->VariableAccessed(name, cmVariableWatch::
+                             UNKNOWN_VARIABLE_READ_ACCESS);
+        }
       }
     }
   return def;
diff --git a/Source/cmVariableWatch.h b/Source/cmVariableWatch.h
index db85371f2..9a978bdfd 100644
--- a/Source/cmVariableWatch.h
+++ b/Source/cmVariableWatch.h
@@ -49,6 +49,7 @@ public:
     {
       VARIABLE_READ_ACCESS,
       UNKNOWN_VARIABLE_READ_ACCESS,
+      ALLOWED_UNKNOWN_VARIABLE_READ_ACCESS,
       VARIABLE_MODIFIED_ACCESS,
       VARIABLE_REMOVED_ACCESS,
       NO_ACCESS