Make this work with Physics::Udunits2

[zmughal]

So that it is easier to install on any platform.

CC: @heikoklein (sent an e-mail about this earlier, but in case that did not reach you).

[heikoklein]

Thanks for the note. An easier installation of Physics::Udunits2 would be nice. Can you send me a PR to the freshly created https://github.com/heikoklein/Physics-Udunits2