Further refinement of the Cygwin problem as suggested by Frederic.

simgear/scene/sky/clouds3d/glut_shapes.c

@@ -48,17 +48,11 @@ OpenGL(TM) is a trademark of Silicon Graphics, Inc.
 #  include <simgear_config.h>
 #endif
 
-
-#ifdef HAVE_WINDOWS_H
-#  include <windows.h>
-#endif
-
+#include "glut_shapes.h"
 
 #include <math.h>
-
 #include <GL/glu.h>
 
-#include "glut_shapes.h"
 
 /* Some <math.h> files do not define M_PI... */
 #ifndef M_PI

simgear/scene/sky/clouds3d/glut_shapes.h

@@ -6,17 +6,12 @@
 #  include <simgear_config.h>
 #endif
 
-
-#ifdef HAVE_WINDOWS_H
-#  include <windows.h>
-#endif
-
-#include <GL/gl.h>
-
 #if defined(WIN32) && !defined(__CYGWIN__) && !defined(__MINGW32__)
 # include <windows.h>
 #endif
 
+#include <GL/gl.h>
+
 #ifdef __cplusplus
 extern "C" {
 #endif