qmake does not expand command-line variables in subdirs

When specifying variable assignmens on the command line, these can themselves contain references to variables defined in the .qmake.conf file or to buildin variables. This works fine for simple projects, but fails if the project uses SUBDIRS.

The attached (admittedly obviously constructed) example contains two projects (works and fails) that are basically identical except for the use of SUBDIRS in fails: the user is expected to provide a variable CONFDIR on the qmake commandline which in the example should be set to $${HIDDENDIR}:

qmake 'CONFDIR=$${HIDDENDIR}'
make

The above shows CONFDIR being set to the expected hidden in the works directory and to ${HIDDENDIR} in the fails case.