Ubuntu 20.04 + LLVM, headless Intel

configure \
    --enable-gpl \
    --enable-nonfree \
    --enable-shared \
    --enable-avresample \
    --enable-frei0r \
    --enable-gcrypt \
    --enable-gmp \
    --enable-gnutls \
    --enable-ladspa \
    --enable-libass \
    --enable-libcaca \
    --enable-libdc1394 \
    --enable-libfdk-aac \
    --enable-libflite \
    --enable-libfontconfig \
    --enable-libfreetype \
    --enable-libfribidi \
    --enable-libgsm \
    --enable-libmp3lame \
    --enable-libopus \
    --enable-libpulse \
    --enable-librsvg \
    --enable-librubberband \
    --enable-librtmp \
    --enable-libsoxr \
    --enable-libspeex \
    --enable-libssh \
    --enable-libtheora \
    --enable-libtwolame \
    --enable-libvorbis \
    --enable-libvpx \
    --enable-libwebp \
    --enable-libx264 \
    --enable-libx265 \
    --enable-opengl \
    --enable-libdrm \
    --enable-vdpau \
    --enable-version3 \
    --enable-libspeex \
    --enable-libxvid \
    --enable-libvpx \
    --extra-cflags='-O3 -mllvm -polly' \
    --cc=clang \
    --cxx=clang++