where are includes?

[Roger Collins]

Roger here on the UnixWare 2.1 platform (must be ancient history for
everyone else).

This must be a silly question but I've checked the install docs and
can't find the answer.  How do I install the includes?  I used
--prefix=$dist --enable-languages="c++".  Then after make bootstrap and
make install, there are only a few obscure header files under $dist.

find $dist | grep iostream

doesn't find anything for example.

What's the secret?

Roger