> Well unless you're lucky enough to have a machine on which to simply
> play with full builds on, then this is the *only* way to do things
> safely.

Are you unfamiliar with DESTDIR?  The source tree will happily use
include files and libraries installed in a private directory.

> (This is, of course, assuming the current contents of
> /usr/src/include were moved into those parts of the tree where they
> have associated responsibilities, and yes, everything does have a
> logical place if you just think about it for a moment.)

I'm not so sure.  Where would you suggest putting
/usr/include/paths.h, for instance?