Attempt to fix build with unusual OpenSSL versions
Since
e3bdb2d92600ed45bd46aaf48309a436a9628218, libpq failed to build on
some platforms because they did not have SSL_clear_options(). Although
mainline OpenSSL introduced SSL_clear_options() after
SSL_OP_NO_COMPRESSION, so the code should have built fine, at least an
old NetBSD version (build farm "coypu" NetBSD 5.1 gcc 4.1.3 PR-
20080704
powerpc) has SSL_OP_NO_COMPRESSION but no SSL_clear_options().
So add a configure check for SSL_clear_options(). If we don't find it,
skip the call. That means on such a platform one cannot *enable* SSL
compression if the built-in default is off, but that seems an unlikely
combination anyway and not very interesting in practice.