Builds on OS X 10.9 disabled

We have several issues making building on OS X 10.9 difficult.

A bug in XCode 6.x have been worked around: https://codereview.qt-project.org/#/c/149903/

But another issue caused this error to suddenly appear in the 5.7 branch: /Users/qt/work/qt/qtwebengine/src/3rdparty/chromium/content/browser/renderer_host/input/synthetic_gesture_target_mac.mm:24:21: error: unknown type name 'NSEventModifierFlags'

And this issue remain in with 49-based:
Module "qt/qtwebengine" (16e81521c1f5a0b3930f480246c71e7caa70396f) did not compile:
/Users/qt/work/qt/qtwebengine/src/3rdparty/chromium/ui/base/cocoa/base_view.mm:174:34: warning: instance method '-stage' not found (return type defaults to 'id') [-Wobjc-method-access]
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.10.sdk/System/Library/Frameworks/AppKit.framework/Headers/NSEvent.h:175:12: note: receiver is instance of class declared here
/Users/qt/work/qt/qtwebengine/src/3rdparty/chromium/ui/base/cocoa/base_view.mm:174:13: error: cannot initialize a variable of type 'NSInteger' (aka 'long') with an rvalue of type 'id'