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
Wrong result is returned when 16-bit value is passed as value.
Also fixes "Warning: using `%edx' instead of `%dx' due to `l' suffix".
Originally committed as revision 14981 to svn://svn.ffmpeg.org/ffmpeg/trunk
