Use CCTK_DECLARE_INIT to declare constants

[eschnett]

Kranc declares a set of constants that may or may not be used in the generated code. If they are not used, they generate compiler warnings about unused variables. This can be prevented by using CCTK_DECLARE_INIT to declare these constants, which uses a system-specific way to suppress warnings about unused variables.

[ianhinder]

It can also be prevented by Kranc not declaring variables which are unused - there is already some code to do this, but it might not be applied universally.