Qt project files and PREFIX variable

PREFIX doesn't mean anything in qmake files. The target for files is done via the target parameter. So if you want to make PREFIX determine the base location, such as /usr/local, you can do do something like this:

isEmpty(PREFIX) {
 PREFIX = /usr/local
}
TARGET = myapp
TARGET.path = $$PREFIX/

The isEmpty(PREFIX) will allow it to be changed during the command line call to qmake, e.g.

qmake PREFIX=/opt

That is INSTALL_ROOT variable on install, try
make install INSTALL_ROOT="your path"