Require GCC 4.6 or later to build glibc.

Require GCC 4.6 or later to build glibc.

As discussed in the thread starting at
<https://sourceware.org/ml/libc-alpha/2014-10/msg00792.html>, and
continuing into November, this patch increases the minimum GCC version
for building glibc to 4.6 (there seemed to be no clear consensus for
4.7).  In particular, this allows us to use #pragma GCC diagnostic for
fine-grained warning control with -Werror (subject to establishing a
suitable policy for that use).  The documentation has a statement, as
requested, about the most recent GCC version tested for building
glibc, and I've updated <https://sourceware.org/glibc/wiki/Release> to
refer to updating that statement.  A NEWS entry is added for this
change, although previous such changes didn't get them.

Tested for x86_64 (testsuite, and that installed shared libraries are
unchanged by this patch).

	* configure.ac (libc_cv_compiler_ok): Require GCC 4.6 or later.
	* configure: Regenerated.
	* manual/install.texi (Tools for Compilation): Document a
	requirement of GCC 4.6 or later and that GCC 4.9 is the newest
	compiler verified to work.
	* INSTALL: Regenerated.