Fix `is_in` compilation error with initializer lists and clang 3.7.1.

Addresses part of issue #121: `template <typename T, int N> bool is_in(T thing, const T (&list)[N])` fails to match against an initializer list second argument with clang-3.7.1 in `modcc/cprinter.cpp`.

* Add overload `template <typename T> bool is_in(T thing, const std::initializer_list<T> list) ` for `is_in` in `modccutil.hpp`. Fixes clang issue and verified to work with a version of gcc as well.