Account for relative jsdoc symlinks.

This happens when you install jsdoc via homebrew on Mac OS X. The jsdoc binary is actually a link to "../Cellar/jsdoc3/3.2.0/bin/jsdoc", which without this patch the BASEPATH line below tries to cd up a level from the current working directory and into a non-existent Cellar directory.
fix shells that don't support [[ … ]] pattern matching (#492)