Fix for ticket https://trac.osgeo.org/grass/ticket/2074 as usggested by Glynn

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@57962 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.mapcalc/map3.c

@@ -521,6 +521,12 @@ void setup_maps(void)
 {
     int i;
 
+    /* We need to reduce the number of worker threads to one, to
+     * avoid that several threads access a single map for reading
+     * at the same time. The raster3d library is not thread safe.
+     * */
+    putenv("WORKERS=1");
+
     for (i = 0; i < num_maps; i++)
 	setup_map(&maps[i]);
 }