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/global_immutable_vars_cpp_runme.rb

@@ -0,0 +1,47 @@
+#!/usr/bin/env ruby
+#
+# C++ version of global_immutable_vars_runme.rb
+#
+
+require 'swig_assert'
+
+require 'global_immutable_vars_cpp'
+
+# first check if all variables can be read
+swig_assert_each_line( <<EOF )
+Global_immutable_vars_cpp::default_mutable_var == 40
+Global_immutable_vars_cpp::global_immutable_var == 41
+Global_immutable_vars_cpp::specific_mutable_var == 42
+Global_immutable_vars_cpp::global_mutable_var == 43
+Global_immutable_vars_cpp::specific_immutable_var == 44
+EOF
+
+# check that all mutable variables can be modified
+swig_assert_each_line( <<EOF )
+Global_immutable_vars_cpp::default_mutable_var = 80
+Global_immutable_vars_cpp::default_mutable_var == 80
+Global_immutable_vars_cpp::specific_mutable_var = 82
+Global_immutable_vars_cpp::specific_mutable_var == 82
+Global_immutable_vars_cpp::global_mutable_var = 83
+Global_immutable_vars_cpp::global_mutable_var == 83
+EOF
+
+# now check that immutable variables cannot be modified
+had_exception = false
+begin
+  Global_immutable_vars_cpp::global_immutable_var = 81
+rescue NoMethodError => e
+  had_exception = true
+end
+swig_assert(had_exception, nil,
+            "Global_immutable_vars_cpp::global_immutable_var is writable (expected to be immutable)")
+
+had_exception = false
+begin
+  Global_immutable_vars_cpp::specific_immutable_var = 81
+rescue NoMethodError => e
+  had_exception = true
+end
+swig_assert(had_exception, nil,
+            "Global_immutable_vars_cpp::specific_immutable_var is writable (expected to be immutable)")
+