Window/Unix: Fix compilation with glxext header versions >=20180525

When mesa updated their headers, they changed the include guard
from __glxext_h_ to __glx_glxext_h_, which breaks compilation
due to conflicting declarations. This commit modifies the preprocessor
directives to allow for compilation with older and newer mesa header
versions.
Fixes: #1472

src/SFML/Window/Unix/GlxExtensions.hpp

@@ -25,11 +25,12 @@
 #ifndef SF_POINTER_C_GENERATED_HEADER_GLXWIN_HPP
 #define SF_POINTER_C_GENERATED_HEADER_GLXWIN_HPP
 
-#ifdef __glxext_h_
+#if defined(__glxext_h_) || defined(__glx_glxext_h_)
 #error Attempt to include glx_exts after including glxext.h
 #endif
 
 #define __glxext_h_
+#define __glx_glxext_h_
 
 #include <X11/Xlib.h>
 #include <X11/Xutil.h>