Added check for the presence of Mesa's header files.
diff --git a/include/wine_gl.h b/include/wine_gl.h
index 3fc7c9c..8a35465 100644
--- a/include/wine_gl.h
+++ b/include/wine_gl.h
@@ -7,7 +7,11 @@
#ifndef __WINE_GL_H
#define __WINE_GL_H
-#ifdef HAVE_MESAGL
+#include "config.h"
+
+#if defined(HAVE_LIBMESAGL) && defined(HAVE_GL_OSMESA_H)
+
+#define HAVE_MESAGL
#undef APIENTRY
#undef CALLBACK
@@ -27,6 +31,10 @@
#define WINAPI __stdcall
#define APIENTRY WINAPI
-#endif /* HAVE_MESAGL */
+#else /* HAVE_LIBMESAGL */
+
+#undef HAVE_MESAGL
+
+#endif /* HAVE_LIBMESAGL */
#endif /* __WINE_GL_H */