Fixes compilation with Mesa >= 18.2.5.

Taken from upstream:

https://github.com/liballeg/allegro5/commit/a40d30e21802ecf5c9382cf34af9b01bd3781e47

diff --git a/include/allegro5/allegro_opengl.h b/include/allegro5/allegro_opengl.h
index 0f86a6768..652dd024e 100644
--- a/include/allegro5/allegro_opengl.h
+++ b/include/allegro5/allegro_opengl.h
@@ -103,10 +103,14 @@

 /* HACK: Prevent both Mesa and SGI's broken headers from screwing us */
 #define __glext_h_
+#define __gl_glext_h_
 #define __glxext_h_
+#define __glx_glxext_h_
 #include <GL/gl.h>
 #undef  __glext_h_
+#undef  __gl_glext_h_
 #undef  __glxext_h_
+#undef  __glx_glxext_h_

 #endif /* ALLEGRO_MACOSX */

diff --git a/include/allegro5/opengl/GLext/glx_ext_defs.h b/include/allegro5/opengl/GLext/glx_ext_defs.h
index 49c502091..fba8aea5d 100644
--- a/include/allegro5/opengl/GLext/glx_ext_defs.h
+++ b/include/allegro5/opengl/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 _ALLEGRO_GLX_VERSION_1_3
--
2.20.0
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<tr><td><span title='2018-06-09 12:02:27 +0200'>2018-06-09</span></td><td><a href='/guix/commit/scripts?id=75e24d7b0e19ab2164aecb340e82d07d2b9714e7'>pull: Install the new Guix in a profile.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-01-28 23:14:55 +0100'>2018-01-28</span></td><td><a href='/guix/commit/scripts?id=63666a348af5ef5190cbe9611c40f2b9188b7967'>guix: Let Emacs detect “scripts/guix.in” appropriate mode.</a><span class='msg-avail'>...</span></td><td>Mathieu Lirzin</td></tr>
<tr><td><span title='2018-01-23 15:01:15 +0100'>2018-01-23</span></td><td><a href='/guix/commit/scripts?id=6f774d481839f87178c5895ac2d661e141f879b8'>guix: Refactor script.</a><span class='msg-avail'>...</span></td><td>Mathieu Lirzin</td></tr>
<tr><td><span title='2018-01-23 10:13:16 +0100'>2018-01-23</span></td><td><a href='/guix/commit/scripts?id=cba386c1295b7054df22dfb4662b1b92f668e2c0'>build: Expand ‘scripts/guix’ at Make time.</a><span class='msg-avail'>...</span></td><td>Mathieu Lirzin</td></tr>