chg: refactor script to fetch external libraries

With this formulation of the script, we only need to change one place in
the script to update or add libraries. (However, the separate .gitignore
and checksums.txt files still need to be updated as well.)

(The unquoted $(basename ...) is safe because we know the basenames
cannot contain spaces – at worst, they would be URL-encoded as %20.)

Change-Id: I47ec8fcb1e1c581df52a9e31f726bcc35fe8f94b