qmake generates incorrect makefiles when using subdirs, ordered build and custom makefile name

Suppose you have a project with subdirectories. The main project file contains:

  TEMPLATE = subdirs
  CONFIG += ordered
  CONFIG += debug_and_release
  MAKEFILE = "Makefile.custom"

When you then create the makefiles with qmake, the custom makefile name is not used in the build rules for the ordered sub-targets.
The result is that when you then start a build with "make -f Makefile.custom debug", the build process changes into the first subdirectory, where it then attempts to execute "make debug" and not "make -f Makefile.custom debug".
This also happens when the makefile name is provided on the qmake command line with the "-o" option.

The bug seems to be in generator/makefile.cpp#2611:

                //don't need the makefile arg if it isn't changed
                QString makefilein;
                if(subtarget->makefile != "$(MAKEFILE)")
                    makefilein = " -f " + subtarget->makefile;

If this is changed to just:

                // always need the makefile arg
                QString makefilein = " -f " + subtarget->makefile;

then it works as intended.