build.sh: modify args handling and default values for the dll suffix

[shawnlaffan]

This simplifies the case for job files but breaks the case where distributions are passed as command line args.

I guess two options are to only use job files or only support one distribution as a command line arg. I'm not sure how common it is to specify jobs on the command line but assume it is not used a lot.

Updates #37

[shawnlaffan]

This should now cover both job files and lists of packages as arguments. In the latter case there is no user control over the dllsuffix, but there never was before anyway.