Detect redefinition of external symbols.
Detect redefinition of external symbols.

An external declaration is a tentative definition, and a file can contain
several tentative definition for a symbol, but when an external
declaration also has an initializer, it is a definition. Due to the order
in the if this error was not catched.

k0gaMSX/kcc,k0gaMSX/kcc,k0gaMSX/scc,k0gaMSX/scc,k0gaMSX/scc