Add correction message to bad linkage error.

src/PNaClTranslator.cpp

Index: src/PNaClTranslator.cpp
diff --git a/src/PNaClTranslator.cpp b/src/PNaClTranslator.cpp
index 7124da08a74b232d530b0aff7e9d57000f64802d..755971d3df5c898ec88d1addedd5ff8cbd2ab892 100644
--- a/src/PNaClTranslator.cpp
+++ b/src/PNaClTranslator.cpp
@@ -473,18 +473,23 @@ private:
     }
   }
 
+  void reportLinkageError(const char *Kind,
+                          const Ice::GlobalDeclaration &Decl) {
+    std::string Buffer;
+    raw_string_ostream StrBuf(Buffer);
+    StrBuf << Kind << " " << Decl.getName()
+           << " has incorrect linkage: " << Decl.getLinkageName();
+    if (Decl.isExternal())
+      StrBuf << "\n  Use flag -allow-externally-defined-symbols to override";
+    Error(StrBuf.str());
+  }
+
   // Converts function declarations into constant value IDs.
   void createValueIDsForFunctions() {
     Ice::GlobalContext *Ctx = getTranslator().getContext();
     for (const Ice::FunctionDeclaration *Func : FunctionDeclarations) {
-      if (!Func->verifyLinkageCorrect(Ctx)) {
-        std::string Buffer;
-        raw_string_ostream StrBuf(Buffer);
-        StrBuf << "Function " << Func->getName()
-               << " has incorrect linkage: " << Func->getLinkageName();
-        Error(StrBuf.str());
-        continue;
-      }
+      if (!Func->verifyLinkageCorrect(Ctx))
+        reportLinkageError("Function", *Func);
       Ice::Constant *C = nullptr;
       if (!isIRGenerationDisabled()) {
         C = getConstantSym(Func->getName(), Func->getSuppressMangling(),
@@ -498,13 +503,8 @@ private:
   void createValueIDsForGlobalVars() {
     Ice::GlobalContext *Ctx = getTranslator().getContext();
     for (const Ice::VariableDeclaration *Decl : *VariableDeclarations) {
-      if (!Decl->verifyLinkageCorrect(Ctx)) {
-        std::string Buffer;
-        raw_string_ostream StrBuf(Buffer);
-        StrBuf << "Global " << Decl->getName()
-               << " has incorrect linkage: " << Decl->getLinkageName();
-        Error(StrBuf.str());
-      }
+      if (!Decl->verifyLinkageCorrect(Ctx))
+        reportLinkageError("Global", *Decl);
       Ice::Constant *C = nullptr;
       if (!isIRGenerationDisabled()) {
         C = getConstantSym(Decl->getName(), Decl->getSuppressMangling(),