Upgrade leptonica to 1.70

This pull request supersedes Homebrew/homebrew#26379, if there is a better way to submit such a pull request please close and let me know how you would prefer I do so.

This incorporates all the working changes from the above referenced issue as well as patch information:

1. Patches jbig2enc to build against leptonica 1.70 for those running stable builds and also adds the option for jbig2enc to pull from HEAD
2. In addition to fixes from the above pull request introduces a patch for the `lept.pc.in` file to make it work properly when building against leptonica and using the pkg-config tool. Said patch has supposedly been accepted for leptonica 1.71 and Works On My Machine™ for my project that depends on leptonica