AutotoolsPM: Fix compile with older gcc/Qt5.6

Change-Id: I60614b87f80443ffa39ed4e4fa59ad0165a63cc8
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/autotoolsprojectmanager/configurestep.cpp

@@ -58,7 +58,7 @@ static QString projectDirRelativeToBuildDir(BuildConfiguration *bc) {
     QString projDirToBuildDir = buildDir.relativeFilePath(
                 bc->target()->project()->projectDirectory().toString());
     if (projDirToBuildDir.isEmpty())
-        return "./";
+        return QString("./");
     if (!projDirToBuildDir.endsWith('/'))
         projDirToBuildDir.append('/');
     return projDirToBuildDir;