build: change equailty test for old bash

Michael used GNU bash, version 1.14.7(1), which apparently does not
seem to know about the double ("==") variant of equality tests.

Reported-by: Michael Teicher <mteicher@gmail.com>
Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>
1 file changed