Why using autotools plugin adds a `--prefix=` parameter whilst not required?

In my snapcraft.yaml I try to build the ffmpeg like that:

    source: https://ffmpeg.org/releases/ffmpeg-4.2.1.tar.bz2
    plugin: autotools
    configflags: ['--enable-libx264','--enable-libmp3lame','--enable-libfdk-aac','--enable-nonfree', '-enable-libx265']
      - libfreetype6-dev 
      - libfribidi-dev 
      - libfontconfig1-dev 
      - yasm 
      - libmp3lame-dev
      - libx264-dev
      - libfdk-aac-dev

But I get the following error:

Building ffmpeg 
./configure --prefix= --enable-libx264 --enable-libmp3lame --enable-libfdk-aac --enable-nonfree -enable-libx265
Unknown option "-enable-libx265".
See ./configure --help for available options.
Failed to run './configure --prefix= --enable-libx264 --enable-libmp3lame --enable-libfdk-aac --enable-nonfree -enable-libx265' for 'ffmpeg': Exited with code 1.
Verify that the part is using the correct parameters and try again.

As far as I understand the --prefix= parameter that is auto appended is the reason why ffmpeg fails to build. Hence I do not want to be auto-apended. How I can acheive that?

this needs a double dash iā€™d guess ā€¦ --enable-libx265