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><th>author</th><td>Tobias Geerinckx-Rice &lt;me@tobias.gr&gt;</td><td class='right'>2021-04-19 12:41:28 +0200</td></tr>
<tr><th>committer</th><td>Tobias Geerinckx-Rice &lt;me@tobias.gr&gt;</td><td class='right'>2021-04-19 19:22:59 +0200</td></tr>
<tr><th>commit</th><td colspan='2' class='oid'><a href='/guix/commit/gnu/packages/monitoring.scm?id=409e20a65c556cad85f132f02564fb9f1f35a4f7'>409e20a65c556cad85f132f02564fb9f1f35a4f7</a> (<a href='/guix/patch/gnu/packages/monitoring.scm?id=409e20a65c556cad85f132f02564fb9f1f35a4f7'>patch</a>)</td></tr>
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<div class='commit-subject'>gnu: inxi-minimal: Update to 3.3.04-1.</div><div class='commit-msg'>* gnu/packages/admin.scm (inxi-minimal): Update to 3.3.04-1.
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