The current implementation was added by commit 5c8693bc (remove
redundant function and eliminate need for strcasecmp, 2003-04-11).  The
code was taken from Graphviz in 2003 from a source file that at the time
was distributed under terms of the Common Public License, Version 1.0.
While the actual content is simple and likely not copyrightable, clarify
the licensing status by simply re-implementing the function from scratch
using another approach.

Change-Id: I44f72b215577af9e3de234b5ef03113c580a3bd6