Enable Namecoin on macOS

Currently, Namecoin is only enabled on GNU/Linux targets; it would be useful to enable it on macOS targets as well, so that macOS users can test the Namecoin support in Nightly without installing a GNU/Linux VM.

(Originally requested by mjgill89 on Reddit.)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information