replace std::string::substr() with operations that do not allocate memory

Modify the original string instead of creating a new copy with substr() when it
is not used for anything else afterwards.

Source/CPack/cmCPackNSISGenerator.cxx

@@ -103,7 +103,7 @@ int cmCPackNSISGenerator::PackageFiles()
         componentName = fileN.substr(0, slash);
 
         // Strip off the component part of the path.
-        fileN = fileN.substr(slash + 1);
+        fileN.erase(0, slash + 1);
       }
     }
     std::replace(fileN.begin(), fileN.end(), '/', '\\');

Source/CPack/cpack.cxx

@@ -85,7 +85,7 @@ int cpackDefinitionArgument(const char* argument, const char* cValue,
     return 0;
   }
   std::string key = value.substr(0, pos);
-  value = value.substr(pos + 1);
+  value.erase(0, pos + 1);
   def->Map[key] = value;
   cmCPack_Log(def->Log, cmCPackLog::LOG_DEBUG,
               "Set CPack variable: " << key << " to \"" << value << "\""