BUILD: SSL: fine guard for SSL_CTX_add_server_custom_ext call

SSL_CTX_add_server_custom_ext is openssl specific function present
since openssl-1.0.2, let us define readable guard for it, not depending
on HA_OPENSSL_VERSION

(cherry picked from commit ec609098718b9c1cd803ca57442b2b98c9ba4a16)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
2 files changed