It's possible that EXTRACT_CMD won't be predefined in the near future
in order to support distfiles in multiple formats. We know the post-
extraction tool needs to be tar, so let's specify it directly.
It may be worth looking at the post-extract target here and seeing
if it's really needed. Often one can just change the WRKSRC value to
avoid a custom post-extract taraget.
Approved by: infrastructure improvements blanket
9c4f0764df