directories and use .PATH to lookup the source files.
(Formerly, the libs were built from the source dirs, with MAKEOBJDIR
set to the compilation directory.)
This solves 2 problems:
-"mkdep" and "make" are now consistent about the file lookup, this fixes
bad interactions with amd reported in PR bin/7374 (Arne Juul) and
lossage reported by Andrew Gillham ("obj" dirs and relative paths still
don't work well together)
-kernel compile trees can be moved around without forcing a new
"make depend" - fixing PR kern/4021 by Martin Husemann
192b3c733c