Add support for safe feature checking at compile time

Added a qtConfig(feature) function to qmake, and a QT_CONFIG(feature)
macro. These can safely check whether a certain compile time feature of
Qt is enabled or not.

For this to work the feature has to have a publicFeature or
privateFeature output in the configure.json file.

In pro files, please use the qtConfig(feature) test function
instead of checking contains(QT_CONFIG, feature), as the latter
will be unreliable with the upcoming modularization (it requires a
load(qt_module_config) before doing any such checks). Note that
feature names are now lowercase, and identical (except for hyphens
versus underscores currently) in the pro and c++ files.

This makes the logic easier to follow, as we avoid all double negations,
and most importantly, QT_CONFIG and qtConfig are implemented in a
way that you'll get a build error for a mistyped or non-existent
feature. This will also prevent accidental use of a widget feature
in gui in the future.

This gives us complete symmetry between the handling in pro and
c++ files.

Change-Id: I60404f97953724e639ffb6386cce2e8b1e4b735a
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

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