Add support for building with ZSTD.

This commit doesn't actually add anything that uses ZSTD; that will be
done separately. It just puts the basic infrastructure into place.

Jeevan Ladhe, Robert Haas, and Michael Paquier. Reviewed by Justin
Pryzby and Andres Freund.

Discussion: http://postgr.es/m/CA+TgmoatQKGd+8SjcV+bzvw4XaoEwminHjU83yG12+NXtQzTTQ@mail.gmail.com
