This package ensures that the machine has required CPU facilities; users who
attempt installation on an inadequate processor will receive an error message
in preinst.  This is useful if there's no way to make your package support
the base ISA for the architecture.

While in general packages are required to at least try, there are situations
when baseline support is infeasible or a pure waste of time.  No one is
going to run scientific number-crunching software on a pre-SSE2/SSE3
machine.  Or upstream code may use such extensions unconditionally, and
you don't have the resources to fix that yourself.

You should also consider how likely other software is to use your package. 
A fast strong hash is something with many uses, big and small, so it's a
priority to port libraries like libhighwayhash to all CPUs.  On the other
hand, a niche piece of software is unlikely to be repurposed for other uses.


The check is skipped in two cases:
* when the target architecture is not executable at all (an ARM SoC mounted
  into an amd64 machine, etc)
* when ISA_IGNORE environment variable has a non-empty value
