[ruby/fiddle] support for very old libffi

Define `Fiddle::TYPE_VARIADIC` only when `ffi_prep_cif_var` is
available, otherwise skip the test for it.

ext/fiddle/fiddle.c

@@ -227,11 +227,13 @@ Init_fiddle(void)
      */
     rb_define_const(mFiddle, "TYPE_DOUBLE",    INT2NUM(TYPE_DOUBLE));
 
+#ifdef HAVE_FFI_PREP_CIF_VAR
     /* Document-const: TYPE_VARIADIC
      *
      * C type - ...
      */
     rb_define_const(mFiddle, "TYPE_VARIADIC",  INT2NUM(TYPE_VARIADIC));
+#endif
 
     /* Document-const: TYPE_SIZE_T
      *

test/fiddle/test_func.rb

@@ -81,6 +81,9 @@ module Fiddle
     end
 
     def test_snprintf
+      unless Fiddle.const_defined?("TYPE_VARIADIC")
+        skip "libffi doesn't support variadic arguments"
+      end
       if Fiddle::WINDOWS
         snprintf_name = "_snprintf"
       else