Adding specialHTMLTargets to EXPORTED_RUNTIME_METHODS carries the
obligation to actually use it as well; failing to do so makes Emscripten
stop with a reference error on startup.
However, we can't guarantee that Qt will use it in all cases. The
current usage depends on QGuiApplication being used. Application code
could be using QCoreApplication, or no application object at all.
Detect if specialHTMLTargets is present instead, and then enable the code
paths which uses it if that's the case. This means that apps which want
to use e.g. multiple browser windows can opt into support by making sure
EXPORTED_RUNTIME_METHODS contains specialHTMLTargets.
Change-Id: I81105aa01946602fcf593f170e305d7dc9bad3be
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>
86103f3af5