Provide feature-test macros for libpq features added in v17.

Provide feature-test macros for libpq features added in v17.

As per the policy established in commit 6991e774e, invent macros
that can be tested at compile time to detect presence of new libpq
features.  This should make calling code more readable and less
error-prone than checking the libpq version would be (especially
since we don't expose that at compile time; the server version is
an unreliable substitute).

Discussion: https://postgr.es/m/2042418.1724346970@sss.pgh.pa.us