diff --git a/simgear/math/SGMath.hxx b/simgear/math/SGMath.hxx
index 80c23ceb..3770408c 100644
--- a/simgear/math/SGMath.hxx
+++ b/simgear/math/SGMath.hxx
@@ -22,13 +22,10 @@
 
 #include <iosfwd>
 // FIXME, make it compile on IRIX
-#ifdef HAVE_CONFIG_H
-#  include <config.h>
-#endif
-#ifdef HAVE_WINDOWS_H
-#  include <windows.h>
-#endif
 #include <osg/GL>
+#undef GLUT_APIENTRY_DEFINED // GL/glut.h undef APIENTRY when this symbol is defined. osg/GL defines it (?).
+                             // This probably would work if we didn't use plib/pu.h that include GL/glut.h 
+                             //  on its side.
 
 #include "SGMathFwd.hxx"