message: On Ubuntu (/Debian) always use embedded GSL.
diff: @@ -168,6 +168,7 @@ install_deps_ubuntu()
  fi
  fi
  + fetch_and_unpack_gsl
  case $RELEASE in
  "18.04" | "19.04" | "20.04" | "21.04")
  # Older Ubuntu's don't have a recent enough fmt / range-v3, so supply it.
  @@ -178,7 +179,6 @@ install_deps_ubuntu()
  fetch_and_unpack_fmtlib
  fetch_and_unpack_Catch2
  - fetch_and_unpack_gsl
  ;;
  *)
  packages="$packages libfmt-dev librange-v3-dev catch2"
  