cmExportSetMap: Override clear() to delete held resources

Replace the std::map<>::clear() method with one that first deletes
the cmExportSet instances held by each map entry, and then clears.
Otherwise the cmGlobalGenerator::ClearGeneratorMembers added by
commit 5cf1120f (cmGlobalGenerator: Refactor member cleanup between
configures, 2013-11-04) leaks the cmExportSet instances.

Source/cmExportSetMap.h

@@ -18,6 +18,7 @@ class cmExportSet;
 /// A name -> cmExportSet map with overloaded operator[].
 class cmExportSetMap : public std::map<std::string, cmExportSet*>
 {
+  typedef std::map<std::string, cmExportSet*> derived;
 public:
   /** \brief Overloaded operator[].
    *
@@ -26,6 +27,8 @@ public:
    */
   cmExportSet* operator[](const std::string &name);
 
+  void clear();
+
   /// Overloaded destructor deletes all member export sets.
   ~cmExportSetMap();
 };