build: use -pthreads for tests with autotools
build: use -pthreads for tests with autotools

This prevents tests from using non-reentrant implementations of common
APIs (e.g `errno`) when using POSIX threads and building with autotools.
This is consistent with the way tests are builds when using gyp.

The problem was found when investigating tests failures on SmartOS for
one test added by https://github.com/libuv/libuv/pull/640. These
failures were due to that test using POSIX threads and also using a
non-reentrant errno global variable, instead of the reentrant `___errno`
function.

PR-URL: https://github.com/libuv/libuv/pull/1052
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>
Reviewed-By: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso