progs/trivial: Use glew where needed.

Builds on windows now.

progs/trivial/readpixels.c

@@ -3,7 +3,7 @@
  */
 
 
-#define GL_GLEXT_PROTOTYPES
+#include <GL/glew.h>
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
@@ -91,6 +91,8 @@ int main(int argc, char **argv)
    if (argc > 1)
       Zoom = atof(argv[1]);
 
+   glewInit();
+
    Init();
 
    glutReshapeFunc(Reshape);