cmMakefile: store the number of last matches in a CMake var

With PushScope and PopScope, keeping track of another bit of data for
each scope isn't easy. Instead, store it as another CMake variable so it
gets implicitly tracked along with everything else.

This works in a revert of commit
7d674b5f0b.

Source/cmStringCommand.h

@@ -53,8 +53,6 @@ public:
   virtual std::string GetName() const { return "string";}
 
   cmTypeMacro(cmStringCommand, cmCommand);
-  static void ClearMatches(cmMakefile* mf);
-  static void StoreMatches(cmMakefile* mf, cmsys::RegularExpression& re);
 protected:
   bool HandleConfigureCommand(std::vector<std::string> const& args);
   bool HandleAsciiCommand(std::vector<std::string> const& args);