libavutil: add version component accessor macros

Pretty standard macros, these should help libav*
users avoid repeating ver.si.on parsing code,
which aids in compatibility-checking tasks like
identifying FFmpeg from Libav (_MICRO >= 100 check).
Something many are doing since we are not
intercompatible anymore.

Signed-off-by: Reynaldo H. Verdejo Pinochet <reynaldo@osg.samsung.com>