meson.build: Do not look for VNC-related libraries if have_system is not set

When running "./configure --static --disable-system" there is currently
a warning if the static version of libpng is missing:

 WARNING: Static library 'png16' not found for dependency 'libpng', may not
 be statically linked

Since it does not make sense to look for the VNC-related libraries at all
when we're building without system emulator binaries, let's add a check
for have_system here to silence this warning.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210906153939.165567-1-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>