fix Clang < 3.9 compilation

Clang < 3.9 requires const objects without an initialiser to have a
user-declared default constructor. It seems this rule was actually a mistake in
the standard [0], but Clang implemented it at its word. This is fixed in Clang
3.9, but to compile with versions prior to this we need to define a trivial
default constructor for a couple of classes.

  [0]: http://open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#253