Referring to C11 (N1570) 6.7.9/5 Initialization:
If the declaration of an identifier has block scope, and the
identifier has external or internal linkage, the declaration shall
have no initializer for the identifier.
The rule is placed within constraints section, so any conforming compiler should reject the code, that violates it.
The point of extern keyword at the block scope is to declare some existing object from outside scope. It would not make much sense to declare an object and give it some other value at the point of declaration.
The recommended way of declaring external objects is to put their declarations at the file scope (at the top of source code), thus they are easy to spot and manage by maintenance programmer.
