Require GCC 5 or later to build glibc (bug 23993)

[Florian Weimer]

* Joseph Myers:

> 2018-12-18  Joseph Myers  <joseph@codesourcery.com>
>
> 	[BZ #23993]
> 	* configure.ac (libc_cv_compiler_ok): Require GCC 5 or later.
> 	* configure: Regenerated.
> 	* manual/install.texi (Tools for Compilation): Update minimum GCC
> 	version.
> 	* INSTALL: Regenerated.

Patch looks okay to me.

However, can we keep support/ at a GCC 4.9 level for some time?  This
will simplify backporting to glibc 2.27 and 2.28.  I expect to backport
regularly to these branches for about a year, maybe longer, particularly
for glibc 2.28.

Thanks,
Florian