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.

tests_errors/global-redef.ispc

@@ -0,0 +1,4 @@
+// Redefinition of variable "foo" is illegal
+
+int foo;
+int foo;