qmake doesn't interpret space in INCLUDEPATH correctly

qmake doesn't work as described in the following document:
http://doc.trolltech.com/4.6/sql-driver.html#qmysql

Example qmake command line:
qmake "INCLUDEPATH=c:\temp dir"

Linux:
qmake "INCLUDEPATH+=/temp\ dir"

It creates compiling options like: -Ic:\temp -Idir

The following line works:
qmake " INCLUDEPATH=\"c:\temp dir\" "