clock-time: On mingw, avoid buggy clock_gettime from libwinpthread. * m4/clock_time.m4 (gl_CLOCK_TIME): Require AC_CANONICAL_HOST. On native Windows, avoid using clock_getres clock_gettime clock_settime from mingw's libwinpthread. * doc/posix-functions/clock_gettime.texi: Mention the mingw bug.