Require GCC 4.9 or later for building glibc.

This patch makes configure require GCC 4.9 or later for building
glibc, and documents that requirement.  Requiring GCC 4.9 or later
allows use of _Generic (as in tzcode).  It would allow <stdatomic.h>
and _Atomic to be used as well if desired, although we need to avoid
any libatomic dependencies on any platforms.  This patch is explicitly
the minimum to implement a new version requirement, with any
consequent cleanups of conditional code (not in installed headers or
files shared with gnulib etc.) to be done separately.

Tested for x86_64.

	* configure.ac (libc_cv_compiler_ok): Require GCC 4.9 or later.
	* configure: Regenerated.
	* manual/install.texi (Tools for Compilation): Document
	requirement for GCC 4.9 or later.
	* INSTALL: Regenerated.