ClearMatches: Only clear matches which were actually set

ClearMatches was clearing many variables which were never set in the
first place. Instead, store how many matches were made last time and
only clear those. It is moved to the cmMakefile class since it is a
common utility used by multiple commands.

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);