[LyX/master] Avoid bogus warnings when configuring for Qt6

[Enrico Forestieri]

commit 66ef66ca0d287ee108c3b85821f31410b47ff618
Author: Enrico Forestieri <forenr at lyx.org>
Date:   Tue Jun 18 21:56:15 2024 +0200

    Avoid bogus warnings when configuring for Qt6
    
    Recent versions of qmake complain loudly when the locale
    does not explicitley specify the utf8 codeset. Specifically,
    the configure script exports LC_ALL=C and that implies the
    ascii codeset. Previously, this was silently changed to utf8,
    whereas now we get a warning for each qmake invocation.
    So, explicitly set utf8 before calling qmake.
---
 config/qt.m4 | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/config/qt.m4 b/config/qt.m4
index 14ec4e3f9f..6a455f9134 100644
--- a/config/qt.m4
+++ b/config/qt.m4
@@ -389,6 +389,10 @@ dnl Use first suitable qmake in PATH
 	  qt_major=5
 	fi
 	AC_CHECK_PROGS([QT_QMAKE], [qmake-qt$qt_major qmake$qt_major qmake], [], $PATH)
+	case ${host} in
+	*mingw*) ;;
+	*) QT_QMAKE="env LC_ALL=C.UTF-8 $QT_QMAKE" ;;
+	esac
 	AC_MSG_CHECKING([for Qt$qt_major])
 	qtver=`$QT_QMAKE -v | grep -o "Qt version ."`
 	if test "$qtver" = "Qt version $qt_major"; then