wasm: support emsdk >= 1.39.4

Keep using the old/deprecated behavior for the Qt 5.14
series.

Task-number: QTBUG-74601
Change-Id: Icee99803f300dfa0116a4de75f9fb26d1010625d
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

mkspecs/wasm-emscripten/qmake.conf

@@ -36,6 +36,7 @@ EMCC_COMMON_LFLAGS += \
     -s NO_EXIT_RUNTIME=0 \
     -s ERROR_ON_UNDEFINED_SYMBOLS=1 \
     -s EXTRA_EXPORTED_RUNTIME_METHODS=[\"UTF16ToString\",\"stringToUTF16\"] \
+    -s DISABLE_DEPRECATED_FIND_EVENT_TARGET_BEHAVIOR=0 \
     --bind
 
 # The -s arguments can also be used with release builds,