Negative.

On Linux/glibc, normally the _BSD_SOURCE, _SVID_SOURCE, _POSIX_SOURCE are on by default. See /usr/include/features.h. On ~BSD, some long double variants may be missing, or implemented as wrappers to a standard double function (with precision loss). Google around (for manpages and discussions). E.g. erfl() appears on FreeBSD 10.1 manpages, but not on 10.0. And no powl(). The sinhl/coshl/tanhl may be wrappers.

I started up a decade-old FreeBSD 6.0 install; looking in libm.a (and libc.a), there is no erfl. The gcc-3.4.4 recognizes __builtin_erfl(), but this just defers to system erfl, and it fails to link.

So, you're out of luck. Furthermore, (some) tests for availability ought to verify the precision, too.