Add limited support for the KHRONOS_* defines from khrplatform.h.

The only way these get used is when you choose to include epoxy/gl.h
but EGL/egl.h.  I'm not sure that's something I want to actually
support, but piglit happened to do so, and this gets things working.

There's not as much support here as in real khrplatform.h, but then
there's no EGL for windows.  And seriously, compilers support 64-bit
and float types, why would you even build this complexity?

include/epoxy/gl.h

@@ -43,12 +43,19 @@ extern "C" {
 #define __glext_h_
 #endif
 
+#define KHRONOS_SUPPORT_INT64   1
+#define KHRONOS_SUPPORT_FLOAT   1
+#define KHRONOS_APIATTRIBUTES
+
 #ifndef _WIN32
 /* APIENTRY and GLAPIENTRY are not used on Linux or Mac. */
 #define APIENTRY
 #define GLAPIENTRY
 #define EPOXYAPIENTRY
 #define GLAPI
+#define KHRONOS_APIENTRY
+#define KHRONOS_APICALL
+
 #else
 #ifndef APIENTRY
 #define APIENTRY __stdcall
@@ -65,6 +72,10 @@ extern "C" {
 #ifndef GLAPI
 #define GLAPI extern
 #endif
+
+#define KHRONOS_APIENTRY __stdcall
+#define KHRONOS_APICALL __declspec(dllimport) __stdcall
+
 #endif /* _WIN32 */
 
 #ifndef APIENTRYP