Fixed unusable dependencies in `.depend'. The following comment in the

Fixed unusable dependencies in `.depend'.  The following comment in the
code still applies, but the code attached to it had rotted:

# ../Makefile.inc will put an absolute path to our objdir in CFLAGS.
# Prevent mkdep from using it, so that we don't have to give rules for
# aliases of generated headers.

Many other makefiles seem to have the same bug (i.e., spelling "." as
"${.OBJDIR}" or as an even more complicated alias in -I directives).