In C89/90 version of C language all aggregate initializers must consist of constants only. In C language terminology a constant of int type is a literal value, like 10, 20u, 0x1 etc. Enum members are also constants. Variables of const int type are not constants in C. You can't use a const int variable in aggregate initializer. (For this reason, in C language, when you need to declare a named constant you should use either #define or enum, but not const qualifier.)
In C99 this requirement for aggregate initializers was relaxed. It is now OK to use non-constants in aggregate initializers of local objects. However, for static objects (as in your example) the requirement still holds. So, even in C99 you'l' have to either use
#define a 0
or use a named enum constant as suggested in @R..'s answer.
