mtdev: Change to use configure detection instead of requiring source hacking.

This coincidentally fixes a case of accidental BIC in qevdevtouch_p.h, where not
all users would necessarily define USE_MTDEV: having it centralized inside Qt
makes this now, blessedly, impossible.

Change-Id: I196a8f21742830705759aa917a823afdc94ba2b5
Done-with: Michael Brasser <michael.brasser@jollamobile.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>