Update INSTALL to mention latest GCC version successfully used.

[Carlos O'Donell]

If nobody objects I'll install this tomorrow to update the fact that newer
stable GCC can be used to build glibc. I use stable GCC 5.3.1 every day.

diff --git a/INSTALL b/INSTALL
index c70ea9f..eb8d094 100644
--- a/INSTALL
+++ b/INSTALL
@@ -362,7 +362,7 @@ build the GNU C Library:
      GCC 4.7 or higher is required.  In general it is recommended to use
      the newest version of the compiler that is known to work for
      building the GNU C Library, as newer compilers usually produce
-     better code.  As of release time, GCC 4.9.2 is the newest compiler
+     better code.  As of release time, GCC 5.3.1 is the newest compiler
      verified to work to build the GNU C Library.
 
      You can use whatever compiler you like to compile programs that use
---

Cheers,
Carlos.