Implement the _SC_THREAD_SAFE_FUNCTIONS sysconf, returning 1.

_SC_THREAD_SAFE_FUNCTIONS is defined in Posix 1003.1c ("posix threads"),
and is supposed to return 1 if the thread-safe functions option is
supported (*_r() functions). Since we do implement those, we should return
1 for this sysconf.

Boost's system library uses this sysconf, and if it sees it is not
available, restorts to the _r()-less variants, for no good reason.