If the LGPL is too strict for usage in your own software project and you still want packJPG decompression functionality, you may use the unpackJPG source code, which is released under the terms of the BSD.

Download it from here:
unpackJPG v2.5h (2016) (BSD)

unpackJPG is based on packJPG, and offers, decompression wise, the same functionality as packJPG.