Missing MwmUtil.h

https://fossies.org/linux/misc/motif-2.3.8.tar.gz/

Copy Xm folder here.
Engine/H/_/

Creating Code Editor Data I'm getting this error.
Unexpected Symbol.
Line (40): "namespace std{typedef decltype(nullptr) nullptr_t;}"
Source: "/EsenthelEngine-main/Editor/Bin/Engine/_/headers.h"
File: "../Source/Code/CE Parser.cpp"
Line: 1740

The december version works.