Only give compiler hint if not compiling natively

If sysroot is None, it means we want to compile natively (e.g.
for the simulator or for a target device with the same arch)

Signed-off-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>
Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5286