strings: Remove redundant calls to std::string::c_str()

Replacements were detected and performed by the clang tool
remove-cstr-calls on a linux build.

Source/cmSetPropertyCommand.cxx

@@ -204,7 +204,7 @@ bool cmSetPropertyCommand::HandleDirectoryMode()
       }
 
     // The local generators are associated with collapsed paths.
-    dir = cmSystemTools::CollapseFullPath(dir.c_str());
+    dir = cmSystemTools::CollapseFullPath(dir);
 
     // Lookup the generator.
     if(cmLocalGenerator* lg =