only use std::integral_constant, if it conforms to the C++11 standard

C++11 requires a cast operator from integral_constant< T, v > to T. As far as I
could make out, no gcc version provides this cast in <tr1/type_traits> and only
gcc >= 4.6 provides it in <type_traits>. Therefore, the configure check now tests
for the cast operator only in <type_traits>.

Note: Our drop-in replacement conforms to the standard in the above-mentioned
sense.

[[Imported from SVN: r6992]]