Selective use of USE_BZIP2

[Tom Hukins]

Is it possible to create a port where one (or more) distfiles is
compressed with bzip2 and one (or more) is compressed with gzip?

Specifically, I would like to amend the pppoa port to fetch the
Sourceforge distfile as a .bz2 instead of a .gz without trying to
extract the proprietary distfile with bunzip2.

So far, I haven't thought of a way to do this.  Let me know if you
have any hints, but if not we can continue fetching the larger gzip
distfile.

Thanks,
Tom