doc/install.texi: Mention contrib/download_prerequisites

Mention ./contrib/download_prerequisites on the "Downloading GCC" page.

	* doc/invoke.texi (Downloading GCC): Mention
	contrib/download_prerequisites script.

From-SVN: r227619

gcc/ChangeLog

+2015-09-10  Markus Trippelsdorf  <markus@trippelsdorf.de>
+
+	* doc/invoke.texi (Downloading GCC): Mention
+	contrib/download_prerequisites script.
+
 2015-09-10  Jakub Jelinek  <jakub@redhat.com>
 
 	PR c++/67523

gcc/doc/install.texi

@@ -565,7 +565,10 @@ components of the binutils you intend to build alongside the compiler
 @file{opcodes}, @dots{}) to the directory containing the GCC sources.
 
 Likewise the GMP, MPFR and MPC libraries can be automatically built
-together with GCC.  Unpack the GMP, MPFR and/or MPC source
+together with GCC.  You may simply run the
+./contrib/download_prerequisites script in the GCC source directory
+to set up everythingu.
+Otherwise unpack the GMP, MPFR and/or MPC source
 distributions in the directory containing the GCC sources and rename
 their directories to @file{gmp}, @file{mpfr} and @file{mpc},
 respectively (or use symbolic links with the same name).