Import mozjpeg-2.1 as wip/mozjpeg.

This project's goal is to reduce the size of JPEG files without
reducing quality or compatibility with the vast majority of the
world's deployed decoders.

The idea is to reduce transfer times for JPEGs on the Web, thus
reducing page load times.

'mozjpeg' is not intended to be a general JPEG library replacement.
It makes tradeoffs that are intended to benefit Web use cases and
focuses solely on improving encoding. It is best used as part of
a Web encoding workflow. For a general JPEG library (e.g. your
system libjpeg), especially if you care about decoding, we recommend
libjpeg-turbo.

TODO:

Decide if this should be integrated in mk/jpeg.buildlink3.mk or
how it should interact with the other jpeg packages.
Add some.