libstdc++: Implement more maintainable <version> header

This commit replaces the ad-hoc logic in <version> with an AutoGen
database that (mostly) declaratively generates a version.h bit which
combines all of the FTM logic across all headers together.

This generated header defines macros of the form __glibcxx_foo,
equivalent to their __cpp_lib_foo variants, according to rules specified
in version.def and, optionally, if __glibcxx_want_foo or
__glibcxx_want_all are defined, also defines __cpp_lib_foo forms with
the same definition.

libstdc++-v3/ChangeLog:

	* include/Makefile.am (bits_freestanding): Add version.h.
	(allcreated): Add version.h.
	(${bits_srcdir}/version.h): New rule.  Regenerates
	version.h out of version.{def,tpl}.
	* include/Makefile.in: Regenerate.
	* include/bits/version.def: New file.  Declares a list of
	all feature test macros, their values and their preconditions.
	* include/bits/version.tpl: New file.  Turns version.def
	into a sequence of #if blocks.
	* include/bits/version.h: New file.  Generated from
	version.def.
	* include/std/version: Replace with a __glibcxx_want_all define
	and bits/version.h include.

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