Work around GCC bug 55856

Work around an internal compiler error of GCC by preventing the
std::pair from being turned into a constexpr.

Feel free to wontfix-close this pull request, I only made it so others might find this solution without having to update their compiler.

Error message: "internal compiler error: in build_data_member_initialization, at cp/semantics.c:5790"
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55856