Attempt to fix build with unusual OpenSSL versions

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.