Fix failure to build QtQuick with LTO

The presence of the inline QTestFontEngine's constructor causes a linker
failure because QFontEngineBox isn't exported. I'd say this is a
compiler bug (GCC 4.9), but it's an easy workaround and a difficult
testcase.

typeinfo for QTestFontEngine: error: undefined reference to 'typeinfo for QFontEngineBox'
vtable for QTestFontEngine: error: undefined reference to 'QFontEngineBox::glyphIndex(unsigned int) const'

Change-Id: I84829d111616977d6f3fcbbb48509d1c7d4f5fa6
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Fix warning about QDeviceDiscovery violating ODR rule

When compiling with GCC 4.9's LTO, the compiler realizes that the class
looks different in two different compilation units and prints a warning.
Adding the necessary #define will make sure that the warning isn't
printed.

It's possible the warning indicates a real problem, if the class
actually got used in those two plugins. I wouldn't know.

QtPlatformSupport/private/.../qdevicediscovery_p.h:66:7: warning: type ‘struct QDeviceDiscovery’ violates one definition rule
.moc/.../qdevicediscovery_p.h:66:7: note: a type with the same name but different layout is defined in another translation unit

Change-Id: I73ca8e553e392b8d368f0deaa318d3e6635d73e1
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>