A falta de pan, buenas son tortas.         

If theres no bread, cakes will do. 

Whenever we are hungry or suffer misfortune, we should be content with whatever is offered and settle for the next best thing - 'beggars can't be choosers'.

Alternative versions of the proverb:

 A falta de pan, galletas.     If there's no bread, have biscuits.

A falta de pan,las tortas son buenas.      For lack of bread, the cakes are good.