Fixed mac build fails
1. ptx_sim.h::473, stack cannot use incomplete type "operand_info" which is a forward declaration. The reason is underlying implementation of stack is deque which need a complete type. It is better to remove forward declaration by break circular dependence for future fixes. It is also benefit unit test
2. shader.h::1334, this hack cannot pass clang. Clang does not allow a array with not a explicit size. Please fix this hack by correct implementation as soon as possible
3. The default parameter causes clang to fail because it frustrate the compiler. This is still under discussion whether such implementation is correct. http://stackoverflow.com/questions/18313509/default-argument-gcc-vs-clang. I changed it to two constructors to avoid confusion.