Create build_extension() to encapsulate the per-extension logic.

Create build_extension() to encapsulate the per-extension logic.
Use @pass_through to hold extra VARIABLE=value assignments.
Add @run to hold optional build-time run prefix.
Use the list form of system for everything.
Always call a target - default to 'all' if not is passed in.
Merge the alternate logic for selecting -MCross from make_ext.pl