Add conditionals for btrfs library versions

Currently the btrfs library API is not versioned and causes build
problems. Add a configure-time conditional check for btrfs/version.h
that defines BTRFS_LIB_VERSION, otherwise assume the value 100.

Use it to fix breakage of btrfs_read_and_process_send_stream in
btrfsprogs 3.16.
Merge pull request #108 from kdave/fix-btrfs-library-api

Add conditionals for btrfs library versions