configure: introduce -logging-backend option

--- src/corelib/global/global.pri
+++ src/corelib/global/global.pri
@@ -44,6 +44,10 @@ if(linux*|hurd*):!cross_compile:!static:!*-armcc* {
    DEFINES += ELF_INTERPRETER=\\\"$$system(LC_ALL=C readelf -l /bin/ls | perl -n -e \'$$prog\')\\\"
 }

+debug_buffer {
+    DEFINES += QT_USE_DEBUG_BUFFER
+}
+
 slog2 {
     LIBS_PRIVATE += -lslog2
     DEFINES += QT_USE_SLOG2
--- src/corelib/global/qlogging.cpp
+++ src/corelib/global/qlogging.cpp
@@ -1499,7 +1499,7 @@ static void qDefaultMessageHandler(QtMsgType type, const QMessageLogContext &con
         return;

     if (!qt_logging_to_console()) {
-#if defined(Q_OS_WIN)
+#if defined(QT_USE_DEBUG_BUFFER)
         logMessage.append(QLatin1Char('\n'));
         OutputDebugString(reinterpret_cast<const wchar_t *>(logMessage.utf16()));
         return;
--- tools/configure/configureapp.cpp
+++ tools/configure/configureapp.cpp
@@ -185,8 +185,8 @@ Configure::Configure(int& argc, char** argv) : verbose(0)
     dictionary[ "QT_EVENTFD" ]      = "auto";
     dictionary[ "QT_CUPS" ]         = "auto";
     dictionary[ "CFG_GCC_SYSROOT" ] = "yes";
-    dictionary[ "LOGGING_BACKEND" ] = "auto";
-    dictionary[ "SLOG2" ]           = "no";
+    dictionary[ "LOGGING_BACKEND" ] = "debug buffer";
+    dictionary["OBSOLETE_LOGGING_BACKEND_ARG"] = "no";
     dictionary[ "QNX_IMF" ]         = "no";
     dictionary[ "PPS" ]             = "no";
     dictionary[ "LGMON" ]           = "no";
@@ -936,9 +936,11 @@ void Configure::parseCmdLine()
         } else if (configCmdLine.at(i) == "-plugin-manifests") {
             dictionary[ "PLUGIN_MANIFESTS" ] = "yes";
         } else if (configCmdLine.at(i) == "-no-slog2") {
-            dictionary[ "SLOG2" ] = "no";
+            dictionary[ "OBSOLETE_LOGGING_BACKEND_ARG" ] = "-no-slog2";
+            dictionary[ "LOGGING_BACKEND" ] = "stderr";
         } else if (configCmdLine.at(i) == "-slog2") {
-            dictionary[ "SLOG2" ] = "yes";
+            dictionary[ "OBSOLETE_LOGGING_BACKEND_ARG" ] = "slog2";
+            dictionary[ "LOGGING_BACKEND" ] = "slog2";
         } else if (configCmdLine.at(i) == "-no-imf") {
             dictionary[ "QNX_IMF" ] = "no";
         } else if (configCmdLine.at(i) == "-imf") {
@@ -1742,8 +1744,8 @@ void Configure::applySpecSpecifics()

         dictionary["DECORATIONS"]           = "default windows styled";
     } else if ((platform() == QNX) || (platform() == BLACKBERRY)) {
+        dictionary["LOGGING_BACKEND"]        = "slog2";
         dictionary["STACK_PROTECTOR_STRONG"] = "auto";
-        dictionary["SLOG2"]                 = "auto";
         dictionary["QNX_IMF"]               = "auto";
         dictionary["PPS"]                   = "auto";
         dictionary["LGMON"]                 = "auto";
@@ -1984,9 +1986,12 @@ bool Configure::displayHelp()
                                                         "by setting the QT_HARFBUZZ environment variable to \"old\".\n"
                                                         "See http://www.harfbuzz.org\n");

+        desc(                "-logging-backend <type>",
+                             qPrintable(QStringLiteral("Send logging output to backend <type>.\n"
+                                                       "Valid values are: debug buffer, slog2, stderr\n"
+                                                       "(Default: %1)\n").arg(dictionary[ "LOGGING_BACKEND"])));
+
         if ((platform() == QNX) || (platform() == BLACKBERRY)) {
-            desc("SLOG2", "yes",  "-slog2",             "Compile with slog2 support.");
-            desc("SLOG2", "no",  "-no-slog2",           "Do not compile with slog2 support.");
             desc("QNX_IMF", "yes",  "-imf",             "Compile with imf support.");
             desc("QNX_IMF", "no",  "-no-imf",           "Do not compile with imf support.");
             desc("PPS", "yes",  "-pps",                 "Compile with PPS support.");
@@ -2558,10 +2563,6 @@ void Configure::autoDetection()
     if (dictionary["STACK_PROTECTOR_STRONG"] == "auto")
         dictionary["STACK_PROTECTOR_STRONG"] = checkAvailability("STACK_PROTECTOR_STRONG") ? "yes" : "no";

-    if ((platform() == QNX || platform() == BLACKBERRY) && dictionary["SLOG2"] == "auto") {
-        dictionary["SLOG2"] = checkAvailability("SLOG2") ? "yes" : "no";
-    }
-
     if ((platform() == QNX || platform() == BLACKBERRY) && dictionary["QNX_IMF"] == "auto") {
         dictionary["QNX_IMF"] = checkAvailability("QNX_IMF") ? "yes" : "no";
     }
@@ -2714,6 +2715,13 @@ bool Configure::verifyConfiguration()
         }
     }

+    if (dictionary["LOGGING_BACKEND"] == "slog2") {
+        if (!checkAvailability("SLOG2")) {
+            cout << "ERROR: slog2 is not available." << endl;
+            dictionary[ "DONE" ] = "error";
+        }
+    }
+
     if (prompt)
         promptKeyPress();

@@ -3517,7 +3525,9 @@ void Configure::generateQConfigPri()
         if (dictionary["CROSS_COMPILE"] == "yes")
             configStream << " cross_compile";

-        if (dictionary[ "SLOG2" ] == "yes")
+        if (dictionary[ "LOGGING_BACKEND" ] == "debug buffer")
+            configStream << " debug_buffer";
+        else if (dictionary[ "LOGGING_BACKEND" ] == "slog2")
             configStream << " slog2";

         if (dictionary[ "QNX_IMF" ] == "yes")
@@ -3880,6 +3890,7 @@ void Configure::displayConfig()
     sout << "QML debugging..............." << dictionary[ "QML_DEBUG" ] << endl;
     sout << "DirectWrite support........." << dictionary[ "DIRECTWRITE" ] << endl;
     sout << "Use system proxies.........." << dictionary[ "SYSTEM_PROXIES" ] << endl;
+    sout << "Logging backend............." << dictionary[ "LOGGING_BACKEND" ] << endl;
     sout << endl;

     sout << "QPA Backends:" << endl;
@@ -3898,7 +3909,6 @@ void Configure::displayConfig()
     sout << "    PCRE support............" << dictionary[ "PCRE" ] << endl;
     sout << "    ICU support............." << dictionary[ "ICU" ] << endl;
     if ((platform() == QNX) || (platform() == BLACKBERRY)) {
-        sout << "    SLOG2 support..........." << dictionary[ "SLOG2" ] << endl;
         sout << "    IMF support............." << dictionary[ "QNX_IMF" ] << endl;
         sout << "    PPS support............." << dictionary[ "PPS" ] << endl;
         sout << "    LGMON support..........." << dictionary[ "LGMON" ] << endl;
@@ -4013,6 +4023,11 @@ void Configure::displayConfig()
              << "will be the same unless you are cross-compiling)." << endl
              << endl;
     }
+    if (dictionary["OBSOLETE_LOGGING_BACKEND_ARG"] != "no") {
+        sout << endl
+             << "NOTE: The " << dictionary["OBSOLETE_LOGGING_BACKEND_ARG"] << " option is obsolete. Use -logging-backend." << endl
+             << endl;
+    }
     if (!dictionary["PREFIX_COMPLAINTS"].isEmpty()) {
         sout << endl
              << dictionary["PREFIX_COMPLAINTS"] << endl