Better integer overflow detection

[mkoeppe]

It seems that 4ti2 relies on the gcc flag -ftrapv to detect integer overflows, but this flag has been
broken for years in gcc, and still is, apparently.

(Reported by Bernard Berthomieu.)

[mkoeppe]

http://www.boost.org/doc/libs/1_58_0/libs/multiprecision/doc/html/boost_multiprecision/tut/ints/cpp_int.html

http://stackoverflow.com/questions/199333/how-to-detect-integer-overflow-in-c-c
http://clang.llvm.org/docs/UsersManual.html#controlling-code-generation
http://clang.llvm.org/docs/LanguageExtensions.html#checked-arithmetic-builtins

Also for reads:
http://stackoverflow.com/questions/17116690/c-read-int-from-istream-detect-overflow