MXS-2057 Do not require systemd libraries

Exclude systemd usage if the library is not installed.
Only excluding what is necessary. This keeps the object size the
same and still compiles most of the code.

server/core/gateway.cc

@@ -27,7 +27,9 @@
 #include <time.h>
 #include <unistd.h>
 #include <getopt.h>
+#ifdef HAVE_SYSTEMD
 #include <systemd/sd-daemon.h>
+#endif
 
 #include <set>
 #include <map>
@@ -1769,12 +1771,14 @@ int main(int argc, char** argv)
         cnf->log_target = MXB_LOG_TARGET_STDOUT;
     }
 
+#ifdef HAVE_SYSTEMD
     // Systemd watchdog. Must be called in the initial thread */
     uint64_t systemd_interval;      // in microseconds
     if (sd_watchdog_enabled(false, &systemd_interval) > 0)
     {
         RoutingWorker::set_watchdog_interval(systemd_interval);
     }
+#endif
 
     if (!daemon_mode)
     {