Don't hardcode libstdc++, fixes build on systems which don't provide it

clang ignores -lstdc++ even if it is absent from the system, giving the
wrong hint to the configure, which proceeds to add it to the list of
libraries.  Then libtool fails because it can't find it.

Why is clang ignoring -lstdc++?  Convenience?

audio/adplug/patches/patch-configure

@@ -0,0 +1,57 @@
+$OpenBSD: patch-configure,v 1.1 2017/10/12 15:04:52 jca Exp $
+
+XXX libstdc++ detected even if not present
+
+Index: configure
+--- configure.orig
++++ configure
+@@ -15013,49 +15013,6 @@ ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
+ 
+ 
+ # Check for needed libraries.
+-{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for main in -lstdc++" >&5
+-$as_echo_n "checking for main in -lstdc++... " >&6; }
+-if test "${ac_cv_lib_stdcpp_main+set}" = set; then :
+-  $as_echo_n "(cached) " >&6
+-else
+-  ac_check_lib_save_LIBS=$LIBS
+-LIBS="-lstdc++  $LIBS"
+-cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+-/* end confdefs.h.  */
+-
+-
+-int
+-main ()
+-{
+-return main ();
+-  ;
+-  return 0;
+-}
+-_ACEOF
+-if ac_fn_cxx_try_link "$LINENO"; then :
+-  ac_cv_lib_stdcpp_main=yes
+-else
+-  ac_cv_lib_stdcpp_main=no
+-fi
+-rm -f core conftest.err conftest.$ac_objext \
+-    conftest$ac_exeext conftest.$ac_ext
+-LIBS=$ac_check_lib_save_LIBS
+-fi
+-{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_lib_stdcpp_main" >&5
+-$as_echo "$ac_cv_lib_stdcpp_main" >&6; }
+-if test "x$ac_cv_lib_stdcpp_main" = x""yes; then :
+-  cat >>confdefs.h <<_ACEOF
+-#define HAVE_LIBSTDC__ 1
+-_ACEOF
+-
+-  LIBS="-lstdc++ $LIBS"
+-
+-else
+-  as_fn_error "libstdc++ not installed" "$LINENO" 5
+-fi
+-
+-
+-
+ if test "x$ac_cv_env_PKG_CONFIG_set" != "xset"; then
+ 	if test -n "$ac_tool_prefix"; then
+   # Extract the first word of "${ac_tool_prefix}pkg-config", so it can be a program name with args.