isnan: Fix compilation error in C++ mode on OpenBSD 7.5-beta. Reported by Christian Weisgerber <naddy@mips.inka.de> in <https://lists.gnu.org/archive/html/bug-gnulib/2024-02/msg00261.html>. * lib/math.in.h (GNULIB_NAMESPACE_LACKS_ISNAN): Define on all platforms with clang ≥ 14.