Avoid certain actions on IMPORTED targets.

As we're iterating over IMPORTED targets now, handle them in
the loop body. The existing behavior is harmless because generally
nothing is done anyway for IMPORTED targets in these code paths,
because they do not have sources for example.

Source/cmGlobalGenerator.cxx

@@ -1379,6 +1379,10 @@ void cmGlobalGenerator::ComputeGeneratorTargetObjects()
     for(cmGeneratorTargetsType::iterator ti = targets.begin();
         ti != targets.end(); ++ti)
       {
+      if (ti->second->Target->IsImported())
+        {
+        continue;
+        }
       cmGeneratorTarget* gt = ti->second;
       gt->ClassifySources();
       gt->LookupObjectLibraries();

Source/cmLocalGenerator.cxx

@@ -261,6 +261,10 @@ void cmLocalGenerator::TraceDependencies()
   for(cmGeneratorTargetsType::iterator t = targets.begin();
       t != targets.end(); ++t)
     {
+    if (t->second->Target->IsImported())
+      {
+      continue;
+      }
     t->second->TraceDependencies();
     }
 }
@@ -545,6 +549,10 @@ void cmLocalGenerator::GenerateTargetManifest()
       {
       continue;
       }
+    if (target.Target->IsImported())
+      {
+      continue;
+      }
     if(configNames.empty())
       {
       target.GenerateTargetManifest(0);