Bug#693481: find_library() in python-webm

[Antoine Martin <antoine@nagafix.co.uk>]

xpra's webm module does it almost right:
1) It shouldn't try to open "libwebp.so";

Oops, fixed.

2) It should treat gnu* (Hurd, kFreeBSD) platforms in the same way as linux2.

Also done.

http://xpra.org/trac/changeset/3709/xpra
(will backport to v0.9.x)

Thanks!
Antoine