Using requires() instead of error() makes the build finish
gracefully when accessibility is not enabled.

This is useful when trying to compile the full Qt5 on platforms
like WinCE.

Change-Id: I4a0f6338c0bd406941b34f4d285b0577f3130dd6
Reviewed-by: Caroline Chao <caroline.chao@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>