Enable binary releases to work on older Linux distros without a static link

This reverts #3914 in favor of a more subtle approach, giving us the benefits of shared base libs and the (relative) portability of full-static.

glibc/libstd++ stubs have been added so that older distros don't see undefined symbols. They are bundled together into an --enable-glibc-back-compat configure switch, as I'm not sure there's any real-world usefulness in separating them.