Merge branch 'ew/autoconf-pthread' into maint

Existing autoconf generated test for the need to link with pthread
library did not check all the functions from pthread libraries;
recent FreeBSD has some functions in libc but not others, and we
mistakenly thought linking with libc is enough when it is not.

* ew/autoconf-pthread:
  configure.ac: stronger test for pthread linkage

diff --cc configure.ac
Simple merge