  Electronic heat capacity was calculated for a mesoscopically disordered
s-wave superconductor treated as a spatial ensemble of domains with a
continuously varying superconducting properties. The domains are assumed to
have sizes larger than the coherence length. Each domain is characterized by a
certain critical temperature. The averaging over the superconducting gap
distribution leads to quadratic temperature dependence of the specific heat at
low temperatures, whereas its anomaly at the critical temperature is
substantially smeared. The results explain well the heat capacity data for
MgB$_{2}$, where a multiple-gap structure is observed.
