configury: propagate _XOPEN_SOURCE cpp defines fully.

* configure.ac (POSIX_EXTRA_CFLAGS): A misnomer...
(POSIX_EXTRA_CPPFLAGS): ...better named like this.
* local.mk (AM_CPPFLAGS): Add POSIX_EXTRA_CPPFLAGS, so addition
preprocessor definitions reach submodule compilation too.
(ext_posix_posix_la_CFLAGS): Remove.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>