genex: Preserve order while evaluating TARGET_OBJECTS

The logic introduced in commit v3.1.0-rc1~688^2~9 (Genex: Evaluate
TARGET_OBJECTS as a normal expression, 2014-02-26) ordered a map
by pointer value and then constructed a list of object files by
iterating over the map.  This is not deterministic.

Since commit v3.1.0-rc1~688^2~5 (cmTarget: Allow any generator
expression in SOURCES property, 2014-03-18) the order produced by the
above-mentioned logic started being used for the actual list of object
files on the link line.  Since it is not deterministic, spurious
re-links occur after re-running CMake simply because the order of
objects changed on the link line.

Fix this by iterating over the original vector of source files instead
of the map.  This has a deterministic order.

Source/cmGeneratorExpressionEvaluator.cxx

@@ -1286,12 +1286,16 @@ static const struct TargetObjectsNode : public cmGeneratorExpressionNode
     std::string obj_dir = gt->ObjectDirectory;
     std::string result;
     const char* sep = "";
-    for(std::map<cmSourceFile const*, std::string>::const_iterator it
-        = mapping.begin(); it != mapping.end(); ++it)
+    for(std::vector<cmSourceFile const*>::const_iterator it
+        = objectSources.begin(); it != objectSources.end(); ++it)
       {
-      assert(!it->second.empty());
+      // Find the object file name corresponding to this source file.
+      std::map<cmSourceFile const*, std::string>::const_iterator
+        map_it = mapping.find(*it);
+      // It must exist because we populated the mapping just above.
+      assert(!map_it->second.empty());
       result += sep;
-      std::string objFile = obj_dir + it->second;
+      std::string objFile = obj_dir + map_it->second;
       cmSourceFile* sf = context->Makefile->GetOrCreateSource(objFile, true);
       sf->SetObjectLibrary(tgtName);
       sf->SetProperty("EXTERNAL_OBJECT", "1");