How to capacity threshold dectector with a simple analogic circuit?
I would use the capacitor as frequency determining element of an oscillator
(i.e. its frequency will depend on the measured proximity) and then determine whether the oscillator frequency is above or below the threshold.

The are probaly several good ways to do the latter, e.g. 


employing a simple f/V converter (e.g. one-shot and low pass filter) plus an comparator or 
(though not an analog solution) using a digital counter that gets periodically reset by a signal of fixed reference frequency and checking whether highest counter bit gets high eventually: