Numerical overflows are possible in some tests for polyxx

[BrunoDutertre]

In test_polynomial.cpp there are large integer constants as in

  CHECK(r == 92254156521408*pow(y,5) - 461361174686720*pow(y,4) + 244807578081920*pow(y,3) - 51113953954560*pow(y,2) + 4803956911040*y - 170197631744);

These constants do not fit in a long on all platforms (because long may be 32bits, for example on Windows).
This causes compilation errors when I try to compile with mingw.