This fixes warnings reported by autoupdate.
* builds/unix/ax_pthread.m4: Replace `as_echo` with `AS_ECHO`.
* builds/unix/configure.raw: Remove obsolete `AC_HEADER_STDC`.
Don't escape back quotes in messages for `AC_MSG_WARN`.
* builds/unix/ax_pthread.m4: New file, taken from 'autoconf-archive'
git repository.
* builds/unix/configure.raw: Check for 'pthread'; also check whether
it works.