Fix windows compilation with new dependency package (libglew only

comes with dynamic libs, not static).

src/graphics/gl_headers.hpp

@@ -1,7 +1,13 @@
 #ifndef GL_HEADER_HPP
 #define GL_HEADER_HPP
 
-#define GLEW_STATIC
+#if defined(WIN32) && !defined(__CYGWIN__)
+   // glew in the binary distribution only comes as dynamic library.
+#  undef GLEW_STATIC
+#else
+#  define GLEW_STATIC
+#endif
+
 extern "C" {
 #include <GL/glew.h>
 }