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='2023-05-19 13:12:29 +0200'>2023-05-19</span></td><td><a href='/guix/commit/gnu/packages/datastructures.scm?id=73d3cb95650691b4209834745c80da0cdcb8f59e'>gnu: zix: Update to 0.3.3-0.a13ae5a.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/datastructures.scm (zix): Update to 0.3.3-0.a13ae5a.
</span></span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2023-05-08 22:28:13 +0200'>2023-05-08</span></td><td><a href='/guix/commit/gnu/packages/datastructures.scm?id=2b99492f5d7800736484f22b723c8e7ce2df134a'>gnu: tllist: Update to 1.1.0.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/datastructures.scm (tllist): Update to 1.1.0.

Signed-off-by: Jelle Licht &lt;jlicht@fsfe.org&gt;
</span></span></td><td>Benjamin</td></tr>
<tr><td><span title='2023-04-07 15:52:54 -0400'>2023-04-07</span></td><td><a href='/guix/commit/gnu/packages/datastructures.scm?id=d2e932cd45bb3bd156b905a0e065ea3509121d68'>gnu: Add zix.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/datastructures.scm (zix): New variable.

Signed-off-by: Maxim Cournoyer &lt;maxim.cournoyer@gmail.com&gt;
Modified-by: Maxim Cournoyer &lt;maxim.cournoyer@gmail.com&gt;
</span></span></td><td>Antero Mejr</td></tr>
<tr><td><span title='2023-02-17 23:58:59 +0100'>2023-02-17</span></td><td><a href='/guix/commit/gnu/packages/datastructures.scm?id=46c06fedbc155e52f9f5173846d531130ab8c8fc'>gnu: Add coucal.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/datastructures.scm (coucal): New variable.
</span></span></td><td>Nicolas Goaziou</td></tr>
<tr><td><span title='2022-08-11 00:21:11 +0200'>2022-08-11</span></td><td><a href='/guix/commit/gnu/packages/datastructures.scm?id=5b5ab17adfe4256de9b9110ba687c6f5cb65a932'>gnu: Add libcuckoo.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/datastructures.scm (libcuckoo): New variable.
</span></span></td><td>Marius Bakke</td></tr>