Fix link error on Linux/musl.

get_nprocs() is a GNU convenience function equivalent to POSIX2008
sysconf(_SC_NPROCESSORS_ONLN); the latter should be available in unistd.h
on any current *nix. (OS X supports this call since 10.5, and FreeBSD
currently supports it. But this commit does not change FreeBSD or OS X
versions.)

On musl (an alternate Linux libc), get_nprocs is not supported, so replace it with the POSIX equivalent.