Instead of trying to open devices /dev/video<0-63> check /dev for
all /dev/videoX devices that are existent in the tree. This allows
finding devices with higher names than 63. E.g. /dev/video100 can
now be found and opened which previously failed.
_GNU_SOURCE #define is introduced for versionsort() which is a GNU
extension.
Fixes#4347.
88aec3b43c