configure: fix setting HAVE_QTDBUS if module is found

The m4 macro `AC_DEFINE()` seems to write the definition into a file
that's later evaluated and from which `config.h` is built. This
temporary file must reside in the proper directory, otherwise
`configure` will only know that `HAVE_QTDBUS` might be defined but not
that it should actually be set.

The function checking for Qt6 uses a different temporary directory for
its checks whether `qmake` can compile with certain modules. For the
invocations it changes the current working directory to the temporary
directory. If `AC_DEFINE()` is used while the working directory is
changed, this will break the mechanism described above.

The solution is to only use `AC_DEFINE()` once the current working
directory's been changed back to the original one.

fixes #3744

NEWS.md

@@ -1,3 +1,11 @@
+# Version ?
+
+## Bug fixes
+
+* configure: fixed setting `HAVE_QTDBUS` properly if the QtDbus module is
+  found. Fixes #3744.
+
+
 # Version 87.0 "Black as the Sky" 2024-09-07
 
 ## New features and enhancements

ac/qt6.m4

@@ -77,6 +77,7 @@ EOT
     fi
   fi
 
+  dbus=0
   modules_to_test=dbus
 
   if test x"$enable_gui" = xno; then
@@ -103,7 +104,7 @@ EOT
       continue
     elif test $qt_module = dbus; then
       qmake_qt_ui="$qmake_qt_ui dbus"
-      AC_DEFINE(HAVE_QTDBUS, 1, [Define if QtDBus is present])
+      dbus=1
     fi
   done
 
@@ -265,6 +266,9 @@ return 0;
   fi
 
   AC_DEFINE(HAVE_QT, 1, [Define if Qt is present])
+  if test "x$dbus" = x1; then
+    AC_DEFINE(HAVE_QTDBUS, 1, [Define if QtDBus is present])
+  fi
   AC_MSG_CHECKING(for Qt 6)
   AC_MSG_RESULT(yes)
   have_qt6=yes