Add some more gl.h definitions that might be used by applications.

These in particular get used by software that was doing
GetProcAddress, and we want to be able to just drop in our gl.h
instead.

include/epoxy/gl.h

@@ -48,6 +48,23 @@ extern "C" {
 #define __glext_h_
 #endif
 
+#ifndef _WIN32
+/* APIENTRY and GLAPIENTRY are not used on Linux or Mac. */
+#define APIENTRY
+#define GLAPIENTRY
+#else
+#ifndef APIENTRY
+#define APIENTRY __stdcall
+#endif
+
+#ifndef GLAPIENTRY
+#define GLAPIENTRY APIENTRY
+#endif
+
+#ifndef GLAPI
+#define GLAPI extern
+#endif
+#endif /* _WIN32 */
 
 #include "epoxy/gl_common.h"
 #include "epoxy/gl_generated.h"