OK. Replying again. This is very frustrating because the configuration of the Unreal Editor an subsequently the configuration that applies to Visual Studio are complex, obtuse and often (in the programming sense) lazy. So the question is not answered.
Here’s the status … if I added the #include <boost/spirit/include/qi.hpp> to my Actor class, it complained it couldn’t load it. So I added to a module file that wasn’t being used as an Unreal class, and that worked — or seemed to work. Now that I examine things, the #include is still underlined by the automation in VS … but the fact that things compiled fooled me into thinking that I has solved things.
It was only after I made a non trivial function using boost and then make a point in my Actor class that called it … that I finally get the “cannot load file” error.
So: How do I add an include path to my project?
(Hint: y’know … this should be a simple question)