compiling under `apple-clang` makes GCV test fail with stochastic approximation

[AlePalu]

Compiling the code under apple-clang, makes any test involving a stochastic approximation of the edf to fail. Possibly related to the way apple-clang sets the seed for the RNG.

[AlePalu]

Solved in d999940. Problem was in the usage of std::default_random_engine, which according to the C++ standard its definition is implementation-defined (see, e.g., here).
Solved by fixing the type of RNG to a 32-bit Mersenne Twister pseudo-random number generator (std::mt19937), which works the same on all platforms and compilers.