There shouldn't be any concerns about portability with $(@D) because:
(1) We only generate this rule iff we know the dirstamp is in a subdir
(so we'd never have a case where $(@D) would expand to the cwd)
(2) We already rely on $(@D) in our depdir code, and have since 2014
We can leverage $(@D) to generate a single pattern rule for all dirstamp
rules. This saves many lines in the output -- normally we create 2 rules
(or 6 lines) per subdir, and projects that use subdirs tend to use them
quite a bit.
In the most extreme & unlikely case (1 subdir, no depdir
