bug#9852: 24.0.90; pthread_sigmask() check is bogus when HAVE_PTHREAD is defined

[Nali Toja]

emacs configure detects (via gnulib) pthread_sigmask even when there is
a stub in libc, marks it broken with

  PTHREAD_SIGMASK_INEFFECTIVE
  PTHREAD_SIGMASK_UNBLOCK_BUG

and links with LIB_PTHREAD during build. What if the version in
LIB_PTHREAD is not broken? gnulib would still wrap it with code
that may not work as expected.
  
forwarding http://www.freebsd.org/cgi/query-pr.cgi?pr=ports/161911
it adds -lpthread to LDFLAGS as a workaround
--
In GNU Emacs 24.0.90.9 (amd64-unknown-freebsd9.9)
 of 2011-10-23 on hidden
configured using `configure  '--without-x'