configure.ac: Test for SSL_library_init
When configuring OpenLDAP using --with-tls=openssl with LibreSSL the configure will fail to detect SSL_export_keying_material_early since LibreSSL doesn't support this function yet. However OpenLDAP doesn't actually use this function so this can be easily solved by checking for SSL_library_init which is a more standard function which both OpenSSL and LibreSSL support which OpenLDAP actually uses in libraries/libldap/tls_o.c.