Fix code generated for Ruby global variables

This commit fixes swig#1653 by creating a Ruby virtual variable
for a C/c++ global variable when SWIG is invoked with the
-globalmodule option.

Examples/test-suite/ruby/ruby_global_immutable_vars_runme.rb

@@ -0,0 +1,51 @@
+#!/usr/bin/env ruby
+#
+# This test program is similar to global_immutable_vars_runme.rb
+# with the difference that the global variables to check are also
+# Ruby global variables (SWIG Ruby option "-globalmodule").
+#
+# Immutable global variables shall throw a NameError exception.
+#
+
+require 'swig_assert'
+
+require 'ruby_global_immutable_vars'
+
+# first check if all variables can be read
+swig_assert_each_line( <<EOF )
+$default_mutable_var == 40
+$global_immutable_var == 41
+$specific_mutable_var == 42
+$global_mutable_var == 43
+$specific_immutable_var == 44
+EOF
+
+# check that all mutable variables can be modified
+swig_assert_each_line( <<EOF )
+$default_mutable_var = 80
+$default_mutable_var == 80
+$specific_mutable_var = 82
+$specific_mutable_var == 82
+$global_mutable_var = 83
+$global_mutable_var == 83
+EOF
+
+# now check that immutable variables cannot be modified
+had_exception = false
+begin
+  $global_immutable_var = 81
+rescue NameError => e
+  had_exception = true
+end
+swig_assert(had_exception, nil,
+            "$global_immutable_var is writable (expected to be immutable)")
+
+had_exception = false
+begin
+  $specific_immutable_var = 81
+rescue NameError => e
+  had_exception = true
+end
+swig_assert(had_exception, nil,
+            "$specific_immutable_var is writable (expected to be immutable)")
+