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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.

--- a/cc1/expr.c

+++ b/cc1/expr.c

@@ -1247,10 +1247,13 @@

 	np = assignop(OINIT, varnode(sym), np);

-	if ((flags & (ISGLOBAL|ISLOCAL|ISPRIVATE)) != 0) {

+	if (flags & ISDEFINED) {

+		errorp("redeclaration of '%s'", sym->name);

+	} else if ((flags & (ISGLOBAL|ISLOCAL|ISPRIVATE)) != 0) {

 		if (!np->right->constant)

 			errorp("initializer element is not constant");

 		emit(OINIT, np);

+		sym->flags |= ISDEFINED;

 	} else if ((flags & (ISEXTERN|ISTYPEDEF)) != 0) {

 		errorp("'%s' has both '%s' and initializer",

 		       sym->name, (flags&ISEXTERN) ? "extern" : "typedef");