Don't use X11 on Mac OS.

https://github.com/KhronosGroup/EGL-Registry/pull/43 made eglplatform.h
include X11 headers on Mac OS, which aren't pre-installed and breaks
the Chrome build.

https://github.com/KhronosGroup/EGL-Registry/issues/52

Change-Id: I73e3c11244a07e269684323ce76cc9700fe9c1aa
Reviewed-on: https://swiftshader-review.googlesource.com/19088
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>

include/EGL/eglplatform.h

diff --git a/include/EGL/eglplatform.h b/include/EGL/eglplatform.h
index 4cba6a1..97c31e2 100644
--- a/include/EGL/eglplatform.h
+++ b/include/EGL/eglplatform.h
@@ -77,7 +77,7 @@
 typedef HBITMAP EGLNativePixmapType;
 typedef HWND    EGLNativeWindowType;
 
-#elif defined(__WINSCW__) || defined(__SYMBIAN32__)  /* Symbian */
+#elif defined(__APPLE__) || defined(__WINSCW__) || defined(__SYMBIAN32__)  /* Symbian */
 
 typedef int   EGLNativeDisplayType;
 typedef void *EGLNativeWindowType;
@@ -110,7 +110,7 @@
 typedef intptr_t EGLNativeWindowType;
 typedef intptr_t EGLNativePixmapType;
 
-#elif defined(__unix__) || defined(__APPLE__)
+#elif defined(__unix__)
 
 /* X11 (tentative)  */
 #include <X11/Xlib.h>