Failure to compile with "--enable-strict" on MacOS

When I configure with --enable-strict, I see this compilation failure on MacOS with the default system Clang.

error: include location '/usr/local/include' is unsafe for cross-compilation [-Werror,-Wpoison-system-directories]

1 error generated.

make[2]: *** [reverse_collator.lo] Error 1

make[1]: *** [all-recursive] Error 1

make: *** [all] Error 2

Not sure what is going on here. -Wpoison-system-directories seems to be about stopping us from using system headers when we're cross-compiling (which we're not) so I'm not sure why we're hitting this.