qtbase-opensource-src-5.9.1 doesn't build with Linux accessibility support

Building qtbase 5.9.1 on Linux always results in a build with accessibility support disabled.

The problem is that the config option for accessibility-atspi-bridge checks for config.atspi, which isn't defined anywhere - it should check for libs.atspi (which is set if pkg-config finds atspi-2) instead.

I've attached a patch that fixes the problem.