Use launcher location to infer ij-dir, similar to path in argv[0]

[jpsacha]

Typically the launcher will be placed in ImageJ directory (ij-dir). Scala Native currently does not provide C side argv[0] to main().

Alternative, possibly more robust ways to determine launcher directory at runtime are considered here:
https://stackoverflow.com/questions/7051844/how-to-find-the-full-path-of-the-c-linux-program-from-within