add include/ruby/backward/cxxanyargs.hpp

Compilation of extension libraries written in C++ are reportedly
broken due to #2404

The root cause of this issue was that the definition of ANYARGS
differ between C and C++, and that of C++ is incompatible with the
updated ones.

We are using the incompatibility against itself.  In C++ two distinct
function prototypes can be overloaded.  We provide the old, ANYARGSed
prototypes in addition to the current granular ones; and let the
older ones warn about types.

eval.c

@@ -947,13 +947,27 @@ rb_need_block(void)
 VALUE
 rb_rescue2(VALUE (* b_proc) (VALUE), VALUE data1,
  VALUE (* r_proc) (VALUE, VALUE), VALUE data2, ...)
+{
+ va_list ap;
+ va_start(ap, data2);
+ return rb_vrescue2(b_proc, data1, r_proc, data2, ap);
+ va_end(ap);
+}
+
+/*!
+ * \copydoc rb_rescue2
+ * \param[in] args exception classes, terminated by 0.
+ */
+VALUE
+rb_vrescue2(VALUE (* b_proc) (VALUE), VALUE data1,
+ VALUE (* r_proc) (VALUE, VALUE), VALUE data2,
+ va_list args)
 {
  enum ruby_tag_type state;
  rb_execution_context_t * volatile ec = GET_EC();
  rb_control_frame_t *volatile cfp = ec->cfp;
  volatile VALUE result = Qfalse;
  volatile VALUE e_info = ec->errinfo;
- va_list args;
 
  EC_PUSH_TAG(ec);
  if ((state = EC_EXEC_TAG()) == TAG_NONE) {
@@ -976,14 +990,12 @@ rb_rescue2(VALUE (* b_proc) (VALUE), VALUE data1,
  int handle = FALSE;
  VALUE eclass;
 
- va_init_list(args, data2);
  while ((eclass = va_arg(args, VALUE)) != 0) {
  if (rb_obj_is_kind_of(ec->errinfo, eclass)) {
  handle = TRUE;
  break;
  }
  }
- va_end(args);
 
  if (handle) {
  result = Qnil;