Add a note about why we have the conditional before including
bsd.compiler.mk. It's so fmake from older 9.x systems still
works (still a supported build config, and having the note here
will let us know when we can cull it more easily).

Also pull in a related change from include to sinclude from
arichardson@'s cross building work, as well as it's companion in
Makefile.inc1 with a note about why we do the odd thing there.

Submitted by: archardson
Differential Revision: https://reviews.freebsd.org/D14241