> Is it possible to compile supped sources without installing the new includes?
> i.e. I want the compiler to use the new includes when compiling the new
> sources, but I don't want to install anything until I'm reasonably sure that
> what I've compiled won't blow me out of the water.  That means my includes
> stay put until the rest of the system changes over.

Yes, and I even thought of doing it right just after smashing the includes
(duh).  I have a spare root partition, so what I eventually did (after a
few false starts) was to create a blank filesystem there, mounted it as /mnt
and set my DESTDIR environment variable to /mnt, used the mtree program
to build a file hierarchy (I used /usr/src/etc/Makefile as a start, but
changed it around a bit not to do a full build), then built and installed
a set of libraries on /mnt, and then set LD_LIBRARY_PATH to /mnt/usr/lib
so I could then build programs using those new libraries.  (This step I
never quite finished.)