wc: treat non breaking space as a word separator

wc: treat non breaking space as a word separator

* src/wc.c (iswnbspace): A new function to match
characters in this class.
(isnbspace): Likewise for single byte charsets.
(main): Initialize posixly_correct from the environment,
to allow disabling honoring NBSP in non C locales.
(wc): Call is[w]nbspace() along with is[w]space.
* bootstrap.conf: Ensure btowc is available.
* tests/misc/wc-nbsp.sh: A new test.
* tests/local.mk: Reference the new test.
* NEWS: Mention the change in behavior.