corrected GL includes

openglcanvas.h

 #ifndef OPENGLCANVAS_H
 #define OPENGLCANVAS_H
 
-#ifndef GL_VERSION_3_1
-    #include <GL/glew.h>
-    #if (!defined GLEW_VERSION_1_5) || (!defined GL_VERSION_3_1)
-        #error OpenGL 3.1 or GLEW 1.5.2 is required
-    #endif
+// GLew might not be needed if a recent version of OpenGL is present
+#include <GL/glew.h>
+#if (!defined GLEW_VERSION_1_5) || (!defined GL_VERSION_3_1)
+    #error GLEW 1.5.2 is required
 #endif
 
-#include <QGLWidget>
-#include <fcntl.h>
-#include <unistd.h>
-#include "chronos.h"
-#include <QTimer>
-
-extern "C" {
-#include <ppm.h>
-#include <pm.h>
-#include <pam.h>
-}
-
 #define GL_GLEXT_PROTOTYPES 1
 #ifdef __APPLE__
   #include<OpenGL/gl.h>
@@ -33,8 +20,19 @@ extern "C" {
   #include <GL/glext.h>
 #endif
 
+#include <QGLWidget>
+#include <QTimer>
 #include <QFile>
 #include <QFileDialog>
+#include <fcntl.h>
+#include <unistd.h>
+#include "chronos.h"
+
+extern "C" {
+#include <ppm.h>
+#include <pm.h>
+#include <pam.h>
+}
 
 class OpenGLCanvas : public QGLWidget
 {