Consistently apply this rule: the guard name is obtained from the
filename by stripping the leading "lib", converting '/' and '.' to
'_' and uppercasing the resulting name. Guard names in the root
directory have to be prefixed by "FFMPEG_".
Originally committed as revision 15120 to svn://svn.ffmpeg.org/ffmpeg/trunk
macro that also contained a description for every field, but, that description
was not being used anywhere. This changes it, to make the description available
as a Doxygen comment. Furthermore, it is now easily parsable by Doxygen, while
before it messed up the documentation.
Originally committed as revision 8254 to svn://svn.ffmpeg.org/ffmpeg/trunk
