Fix global variable code to correctly handle extern declarations.

When we have an "extern" global, now we no longer inadvertently define
storage for it.  Further, we now successfully do define storage when we
encounter a definition following one or more extern declarations.

Issues #215 and #217.

sym.h

@@ -75,7 +75,7 @@ public:
     std::string MangledName() const;
 
     SourcePos pos;            /*!< Source file position where the symbol was defined */
-    const std::string name;   /*!< Symbol's name */
+    std::string name;         /*!< Symbol's name */
     llvm::Value *storagePtr;  /*!< For symbols with storage associated with
                                    them (i.e. variables but not functions),
                                    this member stores a pointer to its