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;