Clang 3.3 complained this line was ambiguous --with-dof-id-bytes=8.

Presumably this is because calling std::min() with mixed types is ambiguous,
but I'm not sure what the compiler thought the two types were supposed to be.
Explicitly casting them both to size_t resolves the issue.
Merge pull request #417 from libMesh/64bit

Clang 3.3 complained this line was ambiguous --with-dof-id-bytes=8.