Fix FTBFS with tagged 4.0~beta1 snapshot, due to gcc-6 gcc-7 libgcc-7-dev.

This is (probably?) 4843ce77 all over
again. The exact error was:

  Removing unwanted packages
  E: Unable to locate package gcc-6
  E: Unable to locate package gcc-7
  E: Unable to locate package libgcc-7-dev
  Reading package lists...
  Building dependency tree...
  Reading state information...
  Package 'binutils-mingw-w64-i686' is not installed, so not removed
  Package 'binutils-mingw-w64-x86-64' is not installed, so not removed
  Package 'binutils-multiarch' is not installed, so not removed
  Package 'binutils-doc' is not installed, so not removed
  E: Unable to locate package gcc-6
  E: Unable to locate package gcc-7
  E: Unable to locate package libgcc-7-dev
  E: config/chroot_local-hooks/98-remove_unwanted_packages failed (exit non-zero). You should check for errors.