COMP: fix compile error

Source/cmLocalVisualStudio7Generator.cxx

@@ -550,11 +550,8 @@ void cmLocalVisualStudio7Generator::FillFlagMapFromCommandFlags(
     {
     option.reserve(strlen(flagTable->commandFlag)+2);
     // first do the - version
-    option.clear();
-    option.insert(static_cast<std::string::size_type>(0), 
-                  static_cast<std::string::size_type>(1), 
-                  '-');
-    option.append(flagTable->commandFlag);
+    option = "-";
+    option += flagTable->commandFlag;
     while(flags.find(option) != flags.npos)
       {
       // replace the flag