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;
 href='/guix/'>guix</a></td><td class='form'><form method='get'>
<input type='hidden' name='id' value='077e813657b46dff06bdb80a9cd1e6411be7eb91'/><input type='hidden' name='showmsg' value='1'/><select name='h' onchange='this.form.submit();'>
<option value='koszko' selected='selected'>koszko</option>
<option value='koszko-scripts'>koszko-scripts</option>
</select> <input type='submit' value='switch'/></form></td></tr>
<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=077e813657b46dff06bdb80a9cd1e6411be7eb91'>refs</a><a class='active' href='/guix/log/gnu/system/pam.scm?showmsg=1'>log</a><a href='/guix/tree/gnu/system/pam.scm?id=077e813657b46dff06bdb80a9cd1e6411be7eb91'>tree</a><a href='/guix/commit/gnu/system/pam.scm?id=077e813657b46dff06bdb80a9cd1e6411be7eb91'>commit</a><a href='/guix/diff/gnu/system/pam.scm?id=077e813657b46dff06bdb80a9cd1e6411be7eb91'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/system/pam.scm'>
<input type='hidden' name='id' value='077e813657b46dff06bdb80a9cd1e6411be7eb91'/><input type='hidden' name='showmsg' value='1'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=077e813657b46dff06bdb80a9cd1e6411be7eb91&amp;showmsg=1'>root</a>/<a href='/guix/log/gnu?id=077e813657b46dff06bdb80a9cd1e6411be7eb91&amp;showmsg=1'>gnu</a>/<a href='/guix/log/gnu/system?id=077e813657b46dff06bdb80a9cd1e6411be7eb91&amp;showmsg=1'>system</a>/<a href='/guix/log/gnu/system/pam.scm?id=077e813657b46dff06bdb80a9cd1e6411be7eb91&amp;showmsg=1'>pam.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/gnu/system/pam.scm?id=077e813657b46dff06bdb80a9cd1e6411be7eb91'>Collapse</a>)</th><th class='left'>Author</th></tr>