this makes it possible to properly exclude entire subprojects based on the
availability of features, rather than stuffing every single source file
with #ifdefs.

the defines are aggregated from the -qconfig <profile>,
-no-feature-<foo> and some other configure flags.

usage:

load(qfeatures)
!contains(QT_DISABLED_FEATURES, textarea): SUBDIRS += textstuff

Task-number: QTBUG-28102
Change-Id: I83400632d64312fa4b907e1318dddfe27c432387
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>
Reviewed-by: Tasuku Suzuki <stasuku@gmail.com>