[LyX/master] Fix detection of private Qt6 headers

[Enrico Forestieri]

commit 76670a6ab9af0809256dd75cda469a4ded9ab75f
Author: Enrico Forestieri <forenr at lyx.org>
Date:   Mon Aug 28 11:03:23 2023 +0200

    Fix detection of private Qt6 headers
    
    The gui-private is a separate module. Private Qt6 GUI headers
    are currently used only with the Windows backend. However they
    are needed to port to Qt6 all functionality that was depending
    on the -extras modules. For example, all code guarded by the
    macro HAVE_QT5_X11_EXTRAS has to be audited and properly replaced
    by Qt6 code. See for example 771f30e9 and 17b24cf4.
---
 config/qt.m4 |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/config/qt.m4 b/config/qt.m4
index b33be23..7e7ffd6 100644
--- a/config/qt.m4
+++ b/config/qt.m4
@@ -414,7 +414,8 @@ EOF1
 QMAKE_EXTRA_VARIABLES = MISSING
 qtHaveModule(core)		{QT += core} else {MISSING += core}
 qtHaveModule(concurrent)	{QT += concurrent} else {MISSING += concurrent}
-qtHaveModule(gui)		{QT += gui gui-private} else {MISSING += gui}
+qtHaveModule(gui)		{QT += gui} else {MISSING += gui}
+qtHaveModule(gui-private)	{QT += gui-private} else {MISSING += gui-private}
 qtHaveModule(svg)		{QT += svg} else {MISSING += svg}
 qtHaveModule(svgwidgets)	{QT += svgwidgets} else {MISSING += svgwidgets}
 qtHaveModule(widgets)		{QT += widgets} else {MISSING += widgets}