Ensure that C++11 checksum is evaluated at compile-time

Wrapping the expression in an std::integral_constant<> seems to do the
right thing, even with no optimization.

Unfortunately, it seems that gcc 4.6 doesn't support user-defined string
literals, so we'll need to wait until we start building with 4.7 before
enabling this.
Merge pull request #91 from bgamari/edge

Ensure that C++11 checksum is evaluated at compile-time