Log Message:
sane-backends: Reorganize PLIST_VARS

pkgsrc changes:
---------------
  * Remove sm3600 variable since it is enabled for any OPSYS.
  * Remove mustek and kvs40xx variables since these backends are built if
    pthread is available. Actually it build on Linux and on other Unix-like
    systems. For Unix-like other than Linux, pthread is not enabled by default
    but we explicitly enable it in Makefile.
  * Bump revision

/!\ pint is built if sys/scanio.h exists. Feel free to add systems that implement it.
/!\ Feel free to reintroduce mustek and kvs40xx for systems that do not implement
    pthread.