allegro4: fix build with mesa 18.2.5

GL/glxext.h include guard was changed in
https://cgit.freedesktop.org/mesa/mesa/commit/include/GL/glxext.h?h=mesa-18.2.5&id=06eb33960e85994bd4ee952395dde6d1762c93b5

pkgs/development/libraries/allegro/allegro4-mesa-18.2.5.patch

@@ -0,0 +1,26 @@
+--- a/addons/allegrogl/include/alleggl.h
++++ b/addons/allegrogl/include/alleggl.h
+@@ -63,9 +63,11 @@ typedef __int64 INT64;
+ /* HACK: Prevent both Mesa and SGI's broken headers from screwing us */
+ #define __glext_h_
+ #define __glxext_h_
++#define __glx_glxext_h_
+ #include <GL/gl.h>
+ #undef  __glext_h_
+ #undef  __glxext_h_
++#undef  __glx_glxext_h_
+ 
+ #endif /* ALLEGRO_MACOSX */
+ 
+--- a/addons/allegrogl/include/allegrogl/GLext/glx_ext_defs.h
++++ b/addons/allegrogl/include/allegrogl/GLext/glx_ext_defs.h
+@@ -1,7 +1,9 @@
+ /* HACK: Prevent both Mesa and SGI's broken headers from screwing us */
+ #define __glxext_h_
++#define __glx_glxext_h_
+ #include <GL/glx.h>
+ #undef __glxext_h_
++#undef __glx_glxext_h_
+ 
+ #ifndef GLX_VERSION_1_3
+ #define AGLX_VERSION_1_3

pkgs/development/libraries/allegro/default.nix

@@ -13,6 +13,7 @@ stdenv.mkDerivation rec {
   };
 
   patches = [
+    ./allegro4-mesa-18.2.5.patch
     ./nix-unstable-sandbox-fix.patch
   ];