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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<div class='path'>path: <a href='/guix/log/?id=c2f928b6bbea79e7a5cf106924a39fa8a76478d9&amp;showmsg=1'>root</a>/<a href='/guix/log/etc?id=c2f928b6bbea79e7a5cf106924a39fa8a76478d9&amp;showmsg=1'>etc</a>/<a href='/guix/log/etc/system-tests.scm?id=c2f928b6bbea79e7a5cf106924a39fa8a76478d9&amp;showmsg=1'>system-tests.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/etc/system-tests.scm?id=c2f928b6bbea79e7a5cf106924a39fa8a76478d9'>Collapse</a>)</th><th class='left'>Author</th></tr>
<tr class='logheader'><td><span title='2022-08-09 15:16:06 +0200'>2022-08-09</span></td><td class='logsubject'><a href='/guix/commit/etc/system-tests.scm?id=5bce4c82422de6beb3ce6120ba1592be898c2b72'>build-system: Add 'channel-build-system'.</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* gnu/ci.scm (channel-build-system, channel-source-&gt;package): Remove.
* gnu/packages/package-management.scm (channel-source-&gt;package): New
procedure, moved from (gnu ci).
* guix/build-system/channel.scm: New file, with code moved from (gnu ci).
* doc/guix.texi (Build Systems): Document it.


</td></tr>
<tr class='logheader'><td><span title='2022-07-10 23:57:14 +0200'>2022-07-10</span></td><td class='logsubject'><a href='/guix/commit/etc/system-tests.scm?id=9fdc4b6c283c5aa5cf10205d87fb2c58b829b9d0'>monads: Add 'mparameterize'.</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* etc/system-tests.scm (mparameterize): Move to...
* guix/monads.scm (mparameterize): ... here.
* tests/monads.scm ("mparameterize"): New test.
* .dir-locals.el (c-mode): Add it.


</td></tr>
<tr class='logheader'><td><span title='2022-04-11 00:25:33 +0200'>2022-04-11</span></td><td class='logsubject'><a href='/guix/commit/etc/system-tests.scm?id=dbde386794cb5f4773b94a20ef585ca0f881544a'>tests: System tests really parameterize 'current-guix-package'.</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
Until now, 'current-guix-package' was parameterized in the wrong
context.  Thus, 'current-guix' would end up building a variant of the
'guix' package instead of the package returned by
'channel-source-&gt;package', which is much less expensive to build.

* etc/system-tests.scm (mparameterize): New macro.
(tests-for-current-guix): Change the 'value' field of each &lt;system-test&gt;
record to parameterize 'current-guix-package' for good.


</td></tr>