@allquixotic, here's the output from the part of the build that throws the errors:
ubuntu@ubuntu-armhf:~/ffmpeg_sources/ffmpeg$ ./configure --prefix="$HOME/ffmpeg_build" \
> --extra-cflags="-I$HOME/ffmpeg_build/include" --extra-ldflags="-L$HOME/ffmpeg_build/lib" \
> --bindir="$HOME/bin" --extra-libs="-ldl" --enable-gpl --enable-libass --enable-libfdk-aac \
> --enable-libmp3lame --enable-libopus --enable-libtheora --enable-libvorbis --enable-libvpx \
> --enable-libx264 --enable-nonfree --enable-x11grab