Require GCC 4.7 or later to build glibc.

Require GCC 4.7 or later to build glibc.

This patch implements a requirement of GCC 4.7 or later to build
glibc.

This was discussed in the thread starting at
<https://sourceware.org/ml/libc-alpha/2015-08/msg00851.html>.
Concerns were expressed by Mike and David.  At
<https://sourceware.org/ml/libc-alpha/2015-10/msg00453.html> I have
provided a 14-patch series showing in outline the cleanups facilitated
by this version requirement, as requested by Mike (this patch is the
first in that series, with the addition of a NEWS entry).  Given the
absence of further concerns or alternative proposals for criteria for
updates to this version requirement as requested in
<https://sourceware.org/ml/libc-alpha/2015-10/msg00065.html>, I am
interpreting this as "absence of sustained opposition" under Carlos's
definition at <https://sourceware.org/glibc/wiki/Consensus> and
proposing this patch for inclusion in glibc.  I'd like to remind
people testing with 4.6 that if they move to testing with GCC 5 then
it will probably be about four years before they need to update the
compiler they use to test glibc again.

Although on the principles of time-based updates I think a move to
requiring binutils 2.23 would be reasonable, I'm not currently aware
of any cleanups that would facilitate so am not proposing that at this
time (but would expect to propose a move to requiring binutils 2.24 in
a year's time, as that brings features such as AVX512 support that
should allow some conditionals to be cleaned up).  If someone thinks a
move to requiring 2.23 would help clean things up for their
architecture, please speak up.  (And in general, I suspect there are
lots of architecture-specific configure tests that could be removed on
the basis of current GCC and binutils version requirements, given how
I've found architecture-independent tests obsolete on the basis of
version requirements going back 20 years.)

Tested for x86_64 and x86 (testsuite, and that installed shared
libraries are unchanged by the patch).

	* configure.ac (libc_cv_compiler_ok): Require GCC 4.7 or later.
	* configure: Regenerated.
	* manual/install.texi (Tools for Compilation): Document
	requirement for GCC 4.7 or later.
	* INSTALL: Regenerated.