fix warning when generating VS 2013 project

/FS forces the compiler to synchronize pdb file writes.
This option is not needed when building with Visual Studio itself.
Still, qmake needs to know it when parsing the compiler flags.

Task-number: QTBUG-36535
Change-Id: Id5b68c4028844e0b95904e08b5121310a4ff13d6
Reviewed-by: Andy Shaw <andy.shaw@digia.com>

qmake/generators/win32/msvc_objectmodel.cpp

@@ -510,6 +510,11 @@ bool VCCLCompilerTool::parseOption(const char* option)
                 BrowseInformation = brAllInfo;
                 BrowseInformationFile = option+3;
                 break;
+            case 'S':
+                if (config->CompilerVersion < NET2013)
+                    found = false;
+                // Ignore this flag. Visual Studio 2013 takes care of this setting.
+                break;
             case 'r':
                 BrowseInformation = brNoLocalSymbols;
                 BrowseInformationFile = option+3;