[LyX/master] Autoconf: warn when using stdlib-debug with Qt6

[Jean-Marc Lasgouttes]

commit 4143dc9202fe7edc072c1d31e1dd89d6f95f3193
Author: Jean-Marc Lasgouttes <lasgouttes at lyx.org>
Date:   Mon Dec 5 19:47:00 2022 +0100

    Autoconf: warn when using stdlib-debug with Qt6
---
 config/qt.m4 |    8 ++++++++
 1 files changed, 8 insertions(+), 0 deletions(-)

diff --git a/config/qt.m4 b/config/qt.m4
index 814d94d..ac8daa2 100644
--- a/config/qt.m4
+++ b/config/qt.m4
@@ -207,6 +207,14 @@ AC_DEFUN([QT_DO_IT_ALL],
 	[AC_MSG_ERROR([LyX requires at least version $1 of Qt. Only version $QTLIB_VERSION has been found.])
 	])
 
+	case $QTLIB_VERSION in
+	6.*) if test $enable_stdlib_debug = "yes" ; then
+		    LYX_WARNING([Compiling LyX with stdlib-debug and Qt6 library may lead to
+   crashes. Consider dropping --enable-stdlib-debug.])
+	     fi;;
+	esac
+
+
 	save_CPPFLAGS=$CPPFLAGS
 	CPPFLAGS="$save_CPPFLAGS $QT_CORE_INCLUDES"
 	AC_CHECK_HEADER(QtGui/qtgui-config.h,