fatal error: 'xwidgets.h' file not found

[yurivict]

Build fails:

c++: warning: optimization flag '-fstrength-reduce' is not supported [-Wignored-optimization-argument]
c++: warning: optimization flag '-fschedule-insns' is not supported [-Wignored-optimization-argument]
In file included from MidiKeyBoard.cpp:22:
./MidiKeyBoard.h:55:10: fatal error: 'xwidgets.h' file not found
   55 | #include "xwidgets.h"
      |          ^~~~~~~~~~~~
1 error generated.

This file is present, just not found:

$ find . -name xwidgets.h
./work/Mamba-2.6/libxputty/xputty/header/xwidgets.h

Version: 2.6
FreeBSD 14.0