OIDN_DIR is pointless

[Netzwerk2]

Setting OIDN_DIR to any directory makes the library compile on my machine :)

I don't know how the linking works, but I have a guess from looking at the build.rs file. To me it looks like rustc can already find the library, but OIDN_DIR needs to be present for the build script to run through. So it should probably be checked if the oidn can be found and if not OIDN_DIR should be checked.