Apply consistently this rule: the guard name is obtained from the
filename stripping the eventual leading "lib", converting '/' and '.'
to '_' and upcasing the resulting name. Headers' guards in the root
dir have to be prefixed by "FFMPEG_".


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@15120 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b