'make install' now installs to $(INSTALL_ROOT)$$QTC_PREFIX/...

This is used for giving the contents of the 7zips an additional prefix.
(previously done by doing an additional copying step in bindistHelper).
QTC_PREFIX can also be used to give Qt Creator a different install path
at qmake time, and defaults to /usr/local on Linux.
On Windows and Mac there is no default for QTC_PREFIX.

Usage: qmake -r QTC_PREFIX=/qtcreator-2.6.0 && make &&
INSTALL_ROOT=/tmp/creator-dist make install

Change-Id: Id30781e14bfdde52531800f22b22e39f0459e806
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>