Require GCC 6.2 or later to build glibc.

Require GCC 6.2 or later to build glibc.

As discussed during development for glibc 2.29, when we increased the
required minimum GCC version for building glibc to GCC 5, working
purely based on the times at which such requirements have been
increased in the past it would be appropriate for glibc 2.30 to
require GCC 6 (matching GCC 4.9 having been required for glibc 2.26).
Naming 6.2 specifically as the minimum version then means a separate
version requirement no longer needs to be specified for powerpc64le.

Thus, this patch increases the minimum to 6.2, removing the
documentation of the separate requirement for powerpc64le.  It does
not remove the powerpc64le configure test, or any __GNUC_PREREQ that
could be removed as not being in installed headers or files shared
with gnulib; I think such cleanups are best done separately.

Tested for x86_64.

	* configure.ac (libc_cv_compiler_ok): Require GCC 6.2 or later.
	* configure: Regenerated.
	* manual/install.texi (Tools for Compilation): Update minimum GCC
	version.
	* INSTALL: Regenerated.