[Bug other/64184] error: '_SC_NPROCESSORS_ONLN' undeclared (first use in this function)

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64184

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It looks like it's used unconditionally even on trunk.

Libstdc++ has an autoconf check for it, see GLIBCXX_CHECK_SC_NPROCESSORS_ONLN
in libstdc++-v3/acinclude.m4