Manage current local generator with automatic var

The cmLocalGenerator::Configure method sets its cmLocalGenerator
instance as the global generator's current local generator during
configuration.  This commit refactors management of the current local
generator to use an automatic variable.  This will allow early returns
from the method.

Source/cmLocalGenerator.cxx

@@ -68,11 +68,30 @@ cmLocalGenerator::~cmLocalGenerator()
   delete this->Makefile;
 }
 
+//----------------------------------------------------------------------------
+class cmLocalGeneratorCurrent
+{
+  cmGlobalGenerator* GG;
+  cmLocalGenerator* LG;
+public:
+  cmLocalGeneratorCurrent(cmLocalGenerator* lg)
+    {
+    this->GG = lg->GetGlobalGenerator();
+    this->LG = this->GG->GetCurrentLocalGenerator();
+    this->GG->SetCurrentLocalGenerator(lg);
+    }
+  ~cmLocalGeneratorCurrent()
+    {
+    this->GG->SetCurrentLocalGenerator(this->LG);
+    }
+};
+
+//----------------------------------------------------------------------------
 void cmLocalGenerator::Configure()
 {
-  cmLocalGenerator* previousLg = 
-                        this->GetGlobalGenerator()->GetCurrentLocalGenerator();
-  this->GetGlobalGenerator()->SetCurrentLocalGenerator(this);
+  // Manage the global generator's current local generator.
+  cmLocalGeneratorCurrent clg(this);
+  static_cast<void>(clg);
 
   // make sure the CMakeFiles dir is there
   std::string filesDir = this->Makefile->GetStartOutputDirectory();
@@ -141,8 +160,6 @@ void cmLocalGenerator::Configure()
   }
 
   this->Configured = true;
-
-  this->GetGlobalGenerator()->SetCurrentLocalGenerator(previousLg);
 }
 
 void cmLocalGenerator::SetupPathConversions()