OpenBSD pch, take 2

[Marc Espie]

Okay, until I find time to relocate precompiled headers, we might
consider that they are totally broken on OpenBSD.

The very nasty thing is that they will *appear* to work. Namely, loading
a pch file will work 95% of the time... and fail whenever the file cannot
be mmaped at the same address due to randomization issues.
So, mark them as completely unreliable.

- is there a simple knob I could set in host-OpenBSD to have PCH always
fail, so that we get predictable results ?  autoconf tests won't work,
since this is a random occurrence...
- barring that, any obvious release notes/documentation where I could write
down that PCH isn't supported under OpenBSD ?