Use -Wno-deprecated-copy only when supported by compiler

This is better than depending on compiler version. In particular,
Apple clang versions do not match regular clang versions.

Note that -Wno-deprecated-copy will always be used with g++, since
this compiler allows to suppress warnings it does not know about
(-Wno-zorg is valid, even though -Wzorg triggers an error).

config/lyxinclude.m4

@@ -409,11 +409,12 @@ if test x$GXX = xyes; then
   dnl Warnings are for preprocessor too
   if test x$enable_warnings = xyes ; then
       AM_CPPFLAGS="$AM_CPPFLAGS -Wall -Wextra"
-      case $gxx_version in
+      dnl Shut off warning -Wdeprecated-copy, which triggers too much
-	  9.*|10.*|clang-10*|clang-11*)
+      dnl note that g++ always accepts -Wno-xxx, even when -Wxxx is an error.
-	      AM_CXXFLAGS="$AM_CXXFLAGS -Wno-deprecated-copy";;
+      AC_LANG_PUSH(C++)
-	  *);;
+      AX_CHECK_COMPILE_FLAG([-Wno-deprecated-copy],
-      esac
+	[AM_CXXFLAGS="$AM_CXXFLAGS -Wno-deprecated-copy"], [], [-Werror])
+      AC_LANG_POP(C++)
     fi
   case $gxx_version in
       2.*|3.*|4.@<:@0-6@:>@) AC_MSG_ERROR([gcc >= 4.7 is required]);;

m4/ax_check_compile_flag.m4

@@ -0,0 +1,53 @@
+# ===========================================================================
+#  https://www.gnu.org/software/autoconf-archive/ax_check_compile_flag.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_CHECK_COMPILE_FLAG(FLAG, [ACTION-SUCCESS], [ACTION-FAILURE], [EXTRA-FLAGS], [INPUT])
+#
+# DESCRIPTION
+#
+#   Check whether the given FLAG works with the current language's compiler
+#   or gives an error.  (Warnings, however, are ignored)
+#
+#   ACTION-SUCCESS/ACTION-FAILURE are shell commands to execute on
+#   success/failure.
+#
+#   If EXTRA-FLAGS is defined, it is added to the current language's default
+#   flags (e.g. CFLAGS) when the check is done.  The check is thus made with
+#   the flags: "CFLAGS EXTRA-FLAGS FLAG".  This can for example be used to
+#   force the compiler to issue an error when a bad flag is given.
+#
+#   INPUT gives an alternative input source to AC_COMPILE_IFELSE.
+#
+#   NOTE: Implementation based on AX_CFLAGS_GCC_OPTION. Please keep this
+#   macro in sync with AX_CHECK_{PREPROC,LINK}_FLAG.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Guido U. Draheim <guidod@gmx.de>
+#   Copyright (c) 2011 Maarten Bosmans <mkbosmans@gmail.com>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved.  This file is offered as-is, without any
+#   warranty.
+
+#serial 6
+
+AC_DEFUN([AX_CHECK_COMPILE_FLAG],
+[AC_PREREQ(2.64)dnl for _AC_LANG_PREFIX and AS_VAR_IF
+AS_VAR_PUSHDEF([CACHEVAR],[ax_cv_check_[]_AC_LANG_ABBREV[]flags_$4_$1])dnl
+AC_CACHE_CHECK([whether _AC_LANG compiler accepts $1], CACHEVAR, [
+  ax_check_save_flags=$[]_AC_LANG_PREFIX[]FLAGS
+  _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $4 $1"
+  AC_COMPILE_IFELSE([m4_default([$5],[AC_LANG_PROGRAM()])],
+    [AS_VAR_SET(CACHEVAR,[yes])],
+    [AS_VAR_SET(CACHEVAR,[no])])
+  _AC_LANG_PREFIX[]FLAGS=$ax_check_save_flags])
+AS_VAR_IF(CACHEVAR,yes,
+  [m4_default([$2], :)],
+  [m4_default([$3], :)])
+AS_VAR_POPDEF([CACHEVAR])dnl
+])dnl AX_CHECK_COMPILE_FLAGS