Fix futex(2) detection and use

Fix futex(2) detection and use

Commit 22e7fc34c44c5ef95abf4fed429f2345410cc82c introduced a regression:
futexes were always disabled and the emulated codepath was always being
used.  That commit was in response to an originally buggy
implementationt hat wrote junk into config.h (but happened to be working
properly).

Fix up the mess and while we're at it, close bug #631231 by including
syscall.h from the correct location and using __NR_futex instead of
SYS_futex.

Closes #631231.