GCC download_prerequisites script edit fails for branch builds

[rouson]

In the latest release of OpenCoarrays, the installer edits GCC's download_prerequisites script if gfortran is built as part of the OpenCoarrays installation. The edit switches the script's download mechanism from wget to whichever one the script has chosen for the given environment (alternatives include curl and ftp). When using OpenCoarrays to install an unreleased branch of GCC, wget gets switch to svn, which doesn't work for downloading the release tar balls of the GCC prerequisites. In such cases, the script needs to use a different download mechanism for the GCC prerequisites from the one it's using for checking out the GCC branch.