Update btrfs-progs to version 3.16

The latest btrfs-progs. Mainly to make the filesystem check tool actually work. This version works also with older btrfs filesystems (tested 3.4 kernel based one).

Had to remove documentation build, as it uses asciidoc which  is not packaged in nemo/mer. Also -O2 optimization flag enabled, as -O1 failed to compile. At quick smoke testing did not find any issues with -O2 built btrfs-progs.