See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86724
Author: Rainer Jung
Date: 2018-07-29

isl 0.20 stopped including other unused header files in their header files.
Therefore,now it's necessary to include the stuff ourselves.
--- .../gcc/graphite.h 2018-01-03 11:03:58.000000000 +0000
+++ .../gcc/graphite.h 2018-07-29 17:42:55.836121000 +0000
@@ -37,6 +37,8 @@
 #include <isl/schedule.h>
 #include <isl/ast_build.h>
 #include <isl/schedule_node.h>
+#include <isl/id.h>
+#include <isl/space.h>

 typedef struct poly_dr *poly_dr_p;
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<tr><td><span title='2020-11-12 09:23:27 -0500'>2020-11-12</span></td><td><a href='/guix/commit/build-aux/update-guix-package.scm?id=3de898b43c1388a9244bdedd2d9f11511c9571d2'>maint: update-guix-package: Optionally add sources to store.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2020-10-25 15:14:31 -0400'>2020-10-25</span></td><td><a href='/guix/commit/build-aux/update-guix-package.scm?id=13a3b9c748a80c0d4c79450e479416480273d2f7'>maint: update-guix-package: Include the git.sv.gnu.org alias.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2020-10-19 14:02:53 -0400'>2020-10-19</span></td><td><a href='/guix/commit/build-aux/update-guix-package.scm?id=5800d2aae2490f4192823323b72d17f2645aeb9e'>maint: update-guix-package: Prevent accidentally breaking guix pull.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2020-05-28 10:21:23 +0200'>2020-05-28</span></td><td><a href='/guix/commit/build-aux/update-guix-package.scm?id=ccd9107ed9e8b918c6fb521cc69ac459fe4257ac'>update-guix-package: Use 'origin-hash'.</a><span class='msg-avail'>...</span></td><td>Vincent Legoll</td></tr>