Cache result of configure_get_default_compiler
Cache result of configure_get_default_compiler

This seems to give a modest performance improvement since it was getting
evaluated over 10 times just to parse any portfile.

Also remove some unneeded uses of subst.

danchr/macports-base,neverpanic/macports-base,macports/macports-base,danchr/macports-base,neverpanic/macports-base,macports/macports-base