[LyX/master] Fixup c52049bb83d: -Werror is needed to test for warning flags

[Jean-Marc Lasgouttes]

commit 37c34c5ca98a31669144a7119068edf96d68261c
Author: Jean-Marc Lasgouttes <lasgouttes at lyx.org>
Date:   Mon Oct 11 14:31:58 2021 +0200

    Fixup c52049bb83d: -Werror is needed to test for warning flags
    
    At least on my old clang++, this is required.
    
    Fixes bug #12391.
---
 config/lyxinclude.m4 |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/config/lyxinclude.m4 b/config/lyxinclude.m4
index 0b41e21..afad2bd 100644
--- a/config/lyxinclude.m4
+++ b/config/lyxinclude.m4
@@ -391,7 +391,7 @@ if test x$GXX = xyes; then
       dnl note that g++ always accepts -Wno-xxx, even when -Wxxx is an error.
       AC_LANG_PUSH(C++)
       AX_CHECK_COMPILE_FLAG([-Wdeprecated-copy],
-	[AM_CXXFLAGS="$AM_CXXFLAGS -Wno-deprecated-copy"])
+	[AM_CXXFLAGS="$AM_CXXFLAGS -Wno-deprecated-copy"], [], [-Werror])
       AC_LANG_POP(C++)
     fi
   case $gxx_version in