root@Emby:/usr/ports/multimedia/ffmpeg/source/FFmpeg # ./configure --help | grep "ass"
  --enable-libass          enable libass subtitles rendering,
                           needed for subtitles and ass filter [no]
  --enable-libzmq          enable message passing via libzmq [no]
  --enable-cross-compile   assume a cross-compiler is used
  --as=AS                  use assembler AS []
  --yasmexe=EXE            use yasm-compatible assembler EXE [yasm]
  --pkg-config-flags=FLAGS pass additional flags to pkgconf []
  --disable-asm            disable all assembly optimizations
  --disable-inline-asm     disable use of inline assembly
  --disable-yasm           disable use of nasm/yasm assembly
  --assert-level=level     0(default), 1 or 2, amount of assertion testing,
                           (group) and PROB the probability associated with
root@Emby:/usr/ports/multimedia/ffmpeg/source/FFmpeg # ./configure  --enable-gpl --enable-nonfree --enable-libass --enable-libfdk-aac --enable-libx264 --enable-libmp3lame --enable-libx265 --enable-openssl --enable-libopus --enable-libfontconfig --enable-libfreetype --enable-libfribidi
ERROR: libass not found using pkg-config

If you think configure made a mistake, make sure you are using the latest
version from Git.  If the latest version fails, report the problem to the
ffmpeg-user@ffmpeg.org mailing list or IRC #ffmpeg on irc.freenode.net.
Include the log file "config.log" produced by configure as this will help
solve the problem.