QMake install problem

[VRonin]

I'm trying to deploy a library using qmake, the aim is to allow to run qmake and make to build the library and then "make install" to move includes and the binaries to a specific folder specified as PREFIX parameter to qmake.
This is what I tried to do, my pro file contains:

installbin.path=$$PREFIX/lib
installbin.files = mylibrary.dll
installbin.files += mylibrary.lib
installbin.files = mylibrary.exp
INSTALLS += installbin

The problem is that, obviously, when I run qmake those files don't exist yet so it won't work.
Any idea how can I fix this?

P.S.
using just

installbin.path=$$PREFIX/lib
INSTALLS += installbin

is not an option as they would be copied to the wrong destination

Thanks in advance