OpenBSD dynamic libraries never link to libc directly.
Instead, they have undefined symbols for all libc functions they use
that ld.so resolves to the libc referred to in the main executable.
Thus, disallowing undefined symbols will always fail
Set HOST_HURD & HOST_UNIX for GNU/Hurd in the makefile-based build
system; the latter variable is important as it will include all the
commit Unix bits.
This is not the most elegant, but will match the SOs in exporting
everything for now. Later we can refine what is public/private to clean
up the interface.
