Source: fix only -Wshorten-64-to-32 warning with explicit cast

Source/cmMachO.cxx

@@ -340,7 +340,8 @@ bool cmMachO::GetInstallName(std::string& install_name)
     if (lc_cmd == LC_ID_DYLIB || lc_cmd == LC_LOAD_WEAK_DYLIB ||
         lc_cmd == LC_LOAD_DYLIB) {
       if (sizeof(dylib_command) < cmd.LoadCommand.size()) {
-        uint32_t namelen = cmd.LoadCommand.size() - sizeof(dylib_command);
+        uint32_t namelen = static_cast<uint32_t>(cmd.LoadCommand.size() -
+                                                 sizeof(dylib_command));
         install_name.assign(&cmd.LoadCommand[sizeof(dylib_command)], namelen);
         return true;
       }