Fix features.h for -Wundef (bug 19212).

features.h is not clean with -Wundef (for the installed header, of
course this only appears with -Wsystem-headers).  In ISO C standards
modes, you get a series of warnings / errors relating to
_POSIX_C_SOURCE and _XOPEN_SOURCE not being defined when tested in
standards mode and uses #undef _GNU_SOURCE to avoid the default
_GNU_SOURCE from libc-symbols.h.  This patch changes the relevant #if
conditionals to avoid these warnings / errors.

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

	[BZ #19212]
	* include/features.h [(_XOPEN_SOURCE - 0) >= 500]: Change
	conditional to [defined _XOPEN_SOURCE && (_XOPEN_SOURCE - 0) >=
	500].
	[_POSIX_C_SOURCE >= 1]: Change conditional to [defined
	_POSIX_C_SOURCE && _POSIX_C_SOURCE >= 1].
	[(_POSIX_C_SOURCE - 0) >= 199309L]: Change conditional to [defined
	_POSIX_C_SOURCE && (_POSIX_C_SOURCE - 0) >= 199309L].
	[(_POSIX_C_SOURCE - 0) >= 199506L]: Change conditional to [defined
	_POSIX_C_SOURCE && (_POSIX_C_SOURCE - 0) >= 199506L].
	[(_POSIX_C_SOURCE - 0) >= 200112L]: Change conditional to [defined
	_POSIX_C_SOURCE && (_POSIX_C_SOURCE - 0) >= 200112L].
	[(_POSIX_C_SOURCE - 0) >= 200809L]: Change conditional to [defined
	_POSIX_C_SOURCE && (_POSIX_C_SOURCE - 0) >= 200809L].