Add option `install.sh --with-gasnet=<loc>`

[bonachea]

When a GASNet installation is not detected, install.sh currently downloads the GASNet sources from a hard-coded URL that lives in https://bitbucket.org/berkeleylab/gasnet/downloads

This is generally reasonable, but BitBucket unfortunately suffers semi-frequent site-wide outages (my release testing is blocked behind one right now). There might also be cases where the user wants to override the choice of URL, for example to select a copy already on the filesystem, or to override the GASNet version without editing the script.

A command-line option to override the choice of GASNet would help mitigate this.