Avoid bogus warnings when configuring for Qt6

Recent versions of qmake complain loudly when the locale
does not explicitly 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

@@ -379,6 +379,10 @@ AC_DEFUN([QT6_QMAKE_CONFIG],
 [
 	dnl Use first suitable qmake in PATH
 	AC_CHECK_PROGS([QT_QMAKE], [qmake-qt6 qmake6], [qmake], $PATH)
+	case ${host} in
+	*mingw*) ;;
+	*) QT_QMAKE="env LC_ALL=C.UTF-8 $QT_QMAKE" ;;
+	esac
 	AC_MSG_CHECKING([for Qt6])
 	qtver=`$QT_QMAKE -v | grep -o "Qt version ."`
 	if test "$qtver" = "Qt version 6"; then

status.24x

@@ -35,6 +35,7 @@ What's new
 
 * BUILD/INSTALLATION
 
+- Avoid bogus warnings when configuring for Qt6.