Emscripten's option for enabling asyncify (-sASYNCIFY) is a link-time
option, which means there is no requirement to have a separate asyncify
build, at least for static builds.
Replace the current QT_HAVE_EMSCRIPTEN_ASYNCIFY compile-time option
with a run-time option which checks if the asyncify API is available.
Keep support for configuring with "-device-option QT_EMSCRIPTEN_ASYNCIFY=1"
for backwards compatibility and for the use case where want asyncify
support to be on by default for a given Qt build.
Enable asyncify for the asyncify_exec example.
Pick-to: 6.4
Change-Id: I301fd7e2d3c0367532c886f4e34b23e1093646ad
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
f347682fd5