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='2020-07-12 23:08:15 +0200'>2020-07-12</span></td><td><a href='/guix/commit/gnu/packages/arcan.scm?id=b0e7b6992f3f845e83cfbca4d700b51dba50b4d5'>gnu: Remove ".git" from "https://github/…/….git".</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-02-16 09:19:06 +0200'>2020-02-16</span></td><td><a href='/guix/commit/gnu/packages/arcan.scm?id=778d6b522ae361767d3cf984a3b182bac7361b7a'>gnu: Remove obsolete bootstrap workaround phases.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2019-03-18 10:51:06 +0100'>2019-03-18</span></td><td><a href='/guix/commit/gnu/packages/arcan.scm?id=b9c88c2292086e6cdb2fe96ea42099b2c9bc4835'>gnu: Add arcan-wayland.</a><span class='msg-avail'>...</span></td><td>Lprndn</td></tr>
<tr><td><span title='2019-03-18 10:51:06 +0100'>2019-03-18</span></td><td><a href='/guix/commit/gnu/packages/arcan.scm?id=38ffa53e2729cd74995a495c3cd4ea4c2f363d2c'>gnu: Add xarcan.</a><span class='msg-avail'>...</span></td><td>Lprndn</td></tr>