wasm: log using emscripten_log() by default

Log using emscripten_log by default, unless
QT_FORCE_STDERR_LOGGING has been set.

It is now possible to log at different levels:

   qDebug() -> Info
   qWarning() -> Warning
   qCritical() -> Error

#ifdef out functions which are now no longer in use
on Q_OS_WASM

Change-Id: I0485e5c070069998a8dfc6759c02bc3b7d6a8a4b
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/corelib/global/qlogging.cpp

@@ -226,6 +226,8 @@ static bool systemHasStderr()
     return true;
 }
 
+#ifndef Q_OS_WASM
+
 /*!
     Returns true if writing to \c stderr will end up in a console/terminal visible to the user.
 
@@ -310,6 +312,8 @@ bool shouldLogToStderr()
 
 using namespace QtPrivate;
 
+#endif // ifndef Q_OS_WASM
+
 /*!
     \class QMessageLogContext
     \inmodule QtCore
@@ -1868,8 +1872,9 @@ static bool wasm_default_message_handler(QtMsgType type,
                                   const QMessageLogContext &,
                                   const QString &formattedMessage)
 {
-    if (shouldLogToStderr())
-        return false; // Leave logging up to stderr handler
+    static bool forceStderrLogging = qEnvironmentVariableIntValue("QT_FORCE_STDERR_LOGGING");
+    if (forceStderrLogging)
+        return false;
 
     int emOutputFlags = EM_LOG_CONSOLE;
     QByteArray localMsg = formattedMessage.toLocal8Bit();