Establish minimum version when liquid-dsp exit() calls removed

[ncorgan]

Currently, liquid-dsp calls exit() when certain input validations fail, and PothosLiquidDSP can trigger these, as it does not perform its own input validation. When this bug is fixed, the next PothosLiquidDSP version should establish a minimum version to prevent running into this.

Relevant issue here: jgaeddert/liquid-dsp#134

[ncorgan]

99bdc92

This allows compilation with LiquidDSP versions before and after the exit() fix, but the auto-generated blocks don't do anything with the return code. This can probably be done by tweaking the mako templates, but the issue still stands.