egl compile error on msys CI runners

[Peter Maydell]

The msys CI runners have started failing to compile with an EGL
related error:

https://gitlab.com/qemu-project/qemu/-/jobs/4015682307

In file included from
C:/GitLab-Runner/builds/qemu-project/qemu/msys64/mingw64/include/epoxy/egl.h:46,
from C:/GitLab-Runner/builds/qemu-project/qemu/include/ui/egl-helpers.h:5,
from ../ui/egl-helpers.c:21:
C:/GitLab-Runner/builds/qemu-project/qemu/msys64/mingw64/include/epoxy/egl_generated.h:11:10:
fatal error: EGL/eglplatform.h: No such file or directory
11 | #include "EGL/eglplatform.h"
| ^~~~~~~~~~~~~~~~~~~
ninja: build stopped: subcommand failed.

Dan says this is because of a msys2 packaging issue:
https://github.com/msys2/MINGW-packages/issues/16492

Can we have a meson test that doesn't try to enable EGL on systems
like this one where it's broken ? Changing the current meson.build
  if cc.has_header('epoxy/egl.h', dependencies: epoxy)
to whatever the meson is for "actually try to compile with the
header" ought to do the trick, I think.

thanks
-- PMM