Make X11 as windowing system opt-in.

Fixes #52
diff --git a/api/EGL/eglplatform.h b/api/EGL/eglplatform.h
index 4cba6a1..0bc2cb9 100644
--- a/api/EGL/eglplatform.h
+++ b/api/EGL/eglplatform.h
@@ -110,7 +110,7 @@
 typedef intptr_t EGLNativeWindowType;
 typedef intptr_t EGLNativePixmapType;
 
-#elif defined(__unix__) || defined(__APPLE__)
+#elif defined(__unix__) || defined(USE_X11)
 
 /* X11 (tentative)  */
 #include <X11/Xlib.h>
@@ -120,6 +120,12 @@
 typedef Pixmap   EGLNativePixmapType;
 typedef Window   EGLNativeWindowType;
 
+#elif defined(__APPLE__)
+
+typedef int   EGLNativeDisplayType;
+typedef void *EGLNativeWindowType;
+typedef void *EGLNativePixmapType;
+
 #elif defined(__HAIKU__)
 
 #include <kernel/image.h>