Make PySide2 optional on windows

* It's already optional on linux due to distributions not necessarily
  carrying packages for it yet. We also make it optional on windows
  since by the same measure it's not a huge problem if it's missing, and
  official builds will include it. This means we don't have to ship the
  binary dependencies

qrenderdoc/Code/pyrenderdoc/PythonContext.cpp

@@ -46,6 +46,14 @@
 PyTypeObject **SbkPySide2_QtCoreTypes = NULL;
 PyTypeObject **SbkPySide2_QtGuiTypes = NULL;
 PyTypeObject **SbkPySide2_QtWidgetsTypes = NULL;
+#else
+
+// for non-windows, this message is displayed at CMake time.
+#ifdef _MSC_VER
+#pragma message( \
+    "Building without PySide2 - Qt will not be accessible in python scripting. See https://github.com/baldurk/renderdoc/wiki/PySide2")
+#endif
+
 #endif
 
 #include <QApplication>