support installation prefix out of the box

qmake has no concept of a base dir for installation, like autoconf-generated configures' --prefix. that means that the user has to choose an arbitrary variable name and surrounding policies, possibly pass it to the source code via a define, etc.
this could be solved by standardizing on a new QMAKE_PREFIX or some such.

changing the default of the existing INSTALL_ROOT is no particularly good option, as it is the equivalent of automake's DESTDIR, which influences only the physical destination of the install targets, but not the final location a package is compiled for. this is irrelevant for fully relocatable executables, but unix apps are typically not relocatable. also, changing the default would break the build systems which did craft their own prefix handling.