progs/tests: compile with SCons and glew

Also get mingw cross-compilation of these tests working
diff --git a/progs/tests/multiwindow.c b/progs/tests/multiwindow.c
index b069bea..6db552d 100644
--- a/progs/tests/multiwindow.c
+++ b/progs/tests/multiwindow.c
@@ -25,6 +25,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <math.h>
+#include <GL/glew.h>
 #include <GL/glut.h>
 
 
@@ -145,6 +146,7 @@
    glutInitWindowSize( 400, 400 );
    glutInitDisplayMode( GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH );
    Window[0] = glutCreateWindow(argv[0]);
+   glewInit();
    glutReshapeFunc( Reshape );
    glutKeyboardFunc( Key );
    glutDisplayFunc( Display0 );
@@ -155,6 +157,7 @@
    glutInitWindowSize( 400, 400 );
    glutInitDisplayMode( GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH );
    Window[1] = glutCreateWindow(argv[0]);
+   glewInit();
    glutReshapeFunc( Reshape );
    glutKeyboardFunc( Key );
    glutDisplayFunc( Display1 );