Fix: (for 32-bit builds): libpolys/misc/intvec.cc should be compiled with "-fwrapv"

This option instructs the compiler to assume that signed arithmetic
overflow of addition, subtraction and multiplication wraps around
using twos-complement representation. This flag enables some
optimizations and disables others.

See http://gcc.gnu.org/onlinedocs/gcc-4.4.6/gcc/Code-Gen-Options.html