Follow-up Comment #1, bug #60811 (project make):
$<, $@, etc are the portable automatic variables. These are standardized by
posix and supported by other unix makes, e.g. sun and ibm makes.
On the other hand, .IMPSRC, etc are not portable.
Introduction of .IMPSRC to gmake cannot improve
Follow-up Comment #6, bug #56701 (project make):
For some reason I always assumed -j without an argument was equivalent to -j
$(nproc).
It certainly can't be made illegal without breaking compatibility with
existing systems that execute make.
Making it equivalent to any number other than
