egl: use unix defines on osx with clang

egl: use unix defines on osx with clang

I also created an bug in Khronos 's bugzilla as you suggested:
https://www.khronos.org/bugzilla/show_bug.cgi?id=1356
I'll let you know if I get feedback from this bug or else where.

Patch with updated error messages:

[PATCH] eglplatform: treat __APPLE__ the same way as __unix__ to handle X11 types

  CC       eglapi.lo
./egldisplay.h:258:19: error: unknown type name 'Display'
_eglGetX11Display(Display *native_display, const EGLint *attrib_list);
eglapi.c:290:4: error: array size is negative
   STATIC_ASSERT(sizeof(void*) == sizeof(nativeDisplay));
eglapi.c:291:25: warning: cast to 'void *' from smaller integer type
   'EGLNativeDisplayType' (aka 'int') [-Wint-to-void-pointer-cast]
   native_display_ptr = (void*) nativeDisplay;
eglapi.c:307:32: error: use of undeclared identifier 'Display'
      dpy = _eglGetX11Display((Display*) native_display, attrib_list);
eglapi.c:776:35: error: use of undeclared identifier 'Window'
      native_window = (void*) (* (Window*) native_window);
eglapi.c:847:35: error: use of undeclared identifier 'Pixmap'
      native_pixmap = (void*) (* (Pixmap*) native_pixmap);

Bugzilla Mesa: https://bugs.freedesktop.org/show_bug.cgi?id=90249
Bugzilla Khronos: https://www.khronos.org/bugzilla/show_bug.cgi?id=1356
Signed-off-by: Julien Isorce <j.isorce@samsung.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>