media-libs/libsfml: various mingw-w64 fixes.

media-libs/libsfml: various mingw-w64 fixes.

Fixes the following:

* cross-compiling for windows does not require x11 and udev, so wrapped in use
  flags

* since we RDEP on build deps, apply a patch accepted for upstream to prevent
  installtion of extlibs when SFML_USE_SYSTEM_DEPS is set

* sets SFML_USE_SYSTEM_DEPS to true (no effect on native builds)

* remove explicit dep on media-libs/mesa as this is provided by virtual/opengl
  in native compilation, and hopefully someday dev-util/mingw64-runtime will be
  an accepted provider for virtual/opengl and virtual/glu, as it provides the
  needed headers and libraries for the mingw-w64 platform.

Closes: https://github.com/gentoo/gentoo/pull/4876
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