install.sh: OpenBSD support to CPU cores check

[mlmikael]

No description provided.

[skaht]

@eric - Believe OSX has forked ties back to OpenBSD. These changes seem logical.

[mlmikael]

@skaht @evoskuil , right, Mac OS X is at least on the user level essentially a BSD derivate. That's reflected in this particular instance where the BSD:s (at least OpenBSD!) and Mac OS X both report to userland how many CPU cores a machine has, namely, through the hw.ncpu sysctl value.

(Supposedly it's the same for FreeBSD and NetBSD (and derivates like DragonBSD etc. though maybe that one would appear as FreeBSD here, duno) but someone else would need to check and introduce that separately.)

[evoskuil]

Incorporated into #147