configure/Mac: Handle errors by xcrun.

xcrun occasionally prompts for a license or outputs errors which
results in:

configure: line 2488: [: : integer expression expected

Check the output and bail out on error.

Change-Id: Ic1ae62b5f19cf87365c38901e98d6b385cdb39a4
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>