https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90834
--- Comment #14 from Iain Sandoe <iains at gcc dot gnu.org> --- other than the desire to locate /usr/local/include in some automatic way, is this still a current issue? I've built (with the workaround for missing __has_xxxxx()) on 10.14 using the 10.15 XC11.0 command line tools, without any issues [other than the missing __has_xxxxx()].
