mkmf.rb: try linking at try_var

To check for variables accessible but not declared.

lib/mkmf.rb

@@ -812,12 +812,20 @@ SRC
   # You should use +have_var+ rather than +try_var+.
   def try_var(var, headers = nil, opt = "", &b)
     headers = cpp_include(headers)
-    try_compile(<<"SRC", opt, &b)
+    try_compile(<<"SRC", opt, &b) or
 #{headers}
 /*top*/
 extern int t(void);
 #{MAIN_DOES_NOTHING 't'}
 int t(void) { const volatile void *volatile p; p = &(&#{var})[0]; return !p; }
+SRC
+    try_link(<<"SRC", opt, &b)
+#{headers}
+/*top*/
+extern int t(void);
+#{MAIN_DOES_NOTHING 't'}
+extern int #{var};
+int t(void) { const volatile void *volatile p; p = &(&#{var})[0]; return !p; }
 SRC
   end
 

test/mkmf/test_have_var.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: false
+require_relative 'base'
+require 'tempfile'
+
+class TestMkmf
+  class TestHaveVar < TestMkmf
+    def test_have_var
+      assert_equal(true, have_var("ruby_version"), MKMFLOG)
+      assert_include($defs, '-DHAVE_RUBY_VERSION')
+    end
+
+    def test_not_have_var
+      assert_equal(false, have_var("rb_vm_something_flag"), MKMFLOG)
+      assert_not_include($defs, '-DHAVE_RB_VM_SOMETHING_FLAG')
+    end
+  end
+end