The log shows the error is:
version:1:1: error: expected unqualified-id
This is a well-known problem resulting from the collision of the project's VERSION text file with the <version> header in C++20-compatible compilers on case-insensitive filesystems. The solution is for the project not to add a -I
flag for the directory that contains the VERSION file or, failing that, renaming the VERSION file to something else, such as VERSION.txt.
This is not a MacPorts- nor macOS-specific problem (though is more prevalent on macOS since Macs usually use a case-insensitive filesystem while other UNIX systems usually do not) and should be reported to the developers of itpp. However itpp seems to be rather dormant so we may have to work on a fix for this ourselves.