pthread: don't assume AC_CANONICAL_HOST, port better to Solaris, etc. * modules/pthread (Depends-on): Add 'extensions', as it defines _POSIX_PTHREAD_SEMANTICS, which is needed on Solaris. (configure.ac-early): New section. * m4/pthread.m4 (gl_PTHREAD_CHECK): Revert previous change, as it is no longer needed.