"Unknown Type Name" build failure(s) — clang 8.0.1

Some type detections are failing or wrong assumptions were made. For example, with strace, old configure are doing some tests on the bool (stdbool.h) declaration but fails on:
conftest.c:84:16: error: initializer element is not a compile-time constant
         int xlcbug = 1 / (&(digs + 5)[-2 + (bool) 1] == &digs[4] ? 1 : -1);
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
# Provided by the macro AC_HEADER_STDBOOL but removed in recent release of autoconf
PackageVersionSupposed error messageFull log Bug report
freebsd-libs 10.3~svn296373-10./libutil.h:62:9: error: unknown type name '__size_t'; did you mean '__ssize_t'? Log
libixion 0.15.0-3../../include/ixion/model_context.hpp:64:20: error: unknown type name 'nullptr_t'; did you mean 'std::nullptr_t'? Log
liborcus 0.15.3-3model_context.hpp:64:20: error: unknown type name 'nullptr_t'; did you mean 'std::nullptr_t'? Log

3 errors