Fix include guards

Macros starting with _ are reserved (see
http://www.gnu.org/software/libc/manual/html_node/Reserved-Names.html).
Fixes #440.
player: Add missing header for ffmpeg1.2

Closes #441.
Makefile: Improve libav selection

See #447.
piano: Fix include guard of crypt.h

Missed that one in 4d8ad7d87072bc1631d0b949930f4869e56b7b99. Fixes #449.
Merge remote-tracking branch 'upstream/master'
note deprication