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
"assignment discards qualifiers from pointer target type" warnings.
blessed by Konstantin Shishkov
Originally committed as revision 11850 to svn://svn.ffmpeg.org/ffmpeg/trunk
