libstdc++: Fix autoconf check for O_NONBLOCK in <fcntl.h>

I misused the AC_CHECK_DECL macro, assuming that it behaved like
AC_CHECK_DECLS and always defined a HAVE_xxx macro if the decl was
found. Instead, the [action-if-found] shell commands are needed to
defined HAVE_O_NONBLOCK explicitly.

libstdc++-v3/ChangeLog:

	* configure.ac: Fix check for O_NONBLOCK.
	* config.h.in: Regenerate.
	* configure: Regenerate.