Modify glx sync to be Linux only.

Mac glxew seems to require X11, which we don't use.

include/gl_utils.h

@@ -24,7 +24,7 @@
 #ifndef GL_UTILS_H
 #define GL_UTILS_H
 
-#ifndef _WIN32
+#ifdef __linux__
 #include <GL/glxew.h>
 #endif
 
@@ -42,7 +42,7 @@ public:
     {
         /// This routine is written for Linux only.  The equivalent functions under Windows would
         /// include <wglext.h> and call wglSwapIntervalEXT
-    #ifndef _WIN32
+    #ifdef __linux__
         Display *dpy = glXGetCurrentDisplay();
         GLXDrawable drawable = glXGetCurrentDrawable();