By "subproject", you mean just another project?

The header file is in a completely different subdirectory? Then put that directory in the include path, or reference them with a relative path from source files in either porject?

I've heard git is better, but I can't imagine what would matter enough to bother with learning something new. Just one person is working on the source (me), so it doesn't seem worth the bother to switch.

I wasn't aware of a ".pri" file. So, I'll have to read about that.

Thanks.