wasm: add support for enabling Asyncify

./configure […] --device-option EMSCRIPTEN_ASYNCIFY=1

Set QT_HAVE_EMSCRIPTEN_ASYNCIFY as a feature flag,
enable optimizations for asyncified debug builds.

Change-Id: I81c887a411780e328aed48ec09ff6b9277c3bccf
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

mkspecs/wasm-emscripten/qmake.conf

@@ -8,6 +8,23 @@ include(../common/clang.conf)
 load(device_config)
 load(emcc_ver)
 
+# Support enabling asyncify by configuring with "-device-option EMSCRIPTEN_ASYNCIFY=1"
+!isEmpty(EMSCRIPTEN_ASYNCIFY): {
+    !equals(EMSCRIPTEN_ASYNCIFY, 1):!equals(EMSCRIPTEN_ASYNCIFY, 0): \
+        message(Error: The value for EMSCRIPTEN_ASYNCIFY must be 0 or 1)
+    equals(EMSCRIPTEN_ASYNCIFY, 1): {
+        QMAKE_CFLAGS += -DQT_HAVE_EMSCRIPTEN_ASYNCIFY
+        QMAKE_CXXFLAGS += -DQT_HAVE_EMSCRIPTEN_ASYNCIFY
+        QMAKE_LFLAGS += -s ASYNCIFY
+
+        # Emscripten recommends building with optimizations when using asyncify
+        # in order to reduce wasm file size, and may also generate broken wasm
+        # (with "wasm validation error: too many locals" type errors) if optimizations
+        # are omitted. Enable optimizations also for debug builds.
+        QMAKE_LFLAGS_DEBUG  += -Os
+    }
+}
+
 EMTERP_FLAGS = \
     -s EMTERPRETIFY=1  \
     -s EMTERPRETIFY_ASYNC=1 \