Make sure that contrib/download_prerequisites is run from correct place

	* download_prerequisites: Make sure that script is run from
	top level source directory.

From-SVN: r227674

contrib/ChangeLog

+2015-09-11  Markus Trippelsdorf  <markus@trippelsdorf.de>
+
+	* download_prerequisites: Make sure that script is run from
+	top level source directory.
+
 2015-08-27  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>
 
 	* update-copyright.py (GCCCmdLine): Add libvtv.

contrib/download_prerequisites

@@ -24,6 +24,11 @@
 # be downloaded.
 GRAPHITE_LOOP_OPT=yes
 
+if [ ! -e gcc/BASE-VER ] ; then
+	echo "You must run this script in the top level GCC source directory."
+	exit 1
+fi
+
 # Necessary to build GCC.
 MPFR=mpfr-2.4.2
 GMP=gmp-4.3.2