configure.exe: Detect MSVC version with environment variable CL cleared

The variable may contain the option /nologo, suppressing the output.

Change-Id: I63eedde10aa7264cf56807a0844cadf6293a1d8c
Task-number: QTBUG-56388
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

tools/configure/environment.cpp

@@ -176,6 +176,7 @@ QString Environment::msvcVersion()
     const QString command = QFile::decodeName(qgetenv("ComSpec"))
         + QLatin1String(" /c ") + QLatin1String(compilerInfo(CC_MSVC2015)->executable)
         + QLatin1String(" /? 2>&1");
+    SetEnvironmentVariable(L"CL", NULL); // May contain /nologo, which suppresses the version.
     QString version = execute(command, &returnValue);
     if (returnValue != 0) {
         cout << "Could not get cl version" << returnValue << qPrintable(version) << '\n';;