Fix some Windows items

Hi,

This MR attempts to fix Windows builds in a few aspects:

• Fix a typo in clutter-events-win32.c, which prevented the code from building.
• Define _USE_MATH_DEFINES in examples/ in the Meson build files so that we can use _M_PI on Visual Studio builds (as well as clang-cl)
• Force-include msvc_recommended_pragmas.h from GLib on Visual Studio, which we are doing on other items that build on top of GLib.
• Avoid returning a value in a void return type function clutter-stage.c, to remove a warning on Visual Studio builds, which is taken to be an error if building against GLib-2.68.x or later.
• Put in the Windows-specific items in clutter-config.h, so that we don't build the Windows backend-specific code in vain.
• Don't set CLUTTER_BACKEND=x11 when running the conformance tests unconditionally.

At this stage the Meson build files are capable of building Clutter on Windows for Visual Studio, at the very least.

The last 2 items allowed most of the conformance tests to run and pass on Windows builds.

With blessings, thank you!