Use acx_pthread.m4 to find pthreads library

In legacy/trac#9495 (moved), we noticed that (it seems) we aren't taking any efforts to compile with threading right on some platforms. Let's fix that by not trying to figure it out ourselves. The acx_pthread.m4 autoconf module is just fine; let's use that in 0.2.7. (It's probably too late to do this in 0.2.6; this is a fiddly part of our build system)