Refactor the creation of imported targets into a macro.

Refactor the creation of imported targets into a macro.

This will make it easier to handle cases where the Qt installation
and the build do not match exactly. For example, on Windows, it
would be possible to do:

  ./configure -debug
  make
  make install
  make release
  make release_install

In which case, both debug and release libraries would be installed
even though it was only configured with -debug. On non-Windows, the
debug and release libraries would overwrite each other, so it is
not necessary to support it.

Similarly, we want to handle cases in the future where (on
non-Windows) both static and shared libraries would be
installed (again, not described with a single build configuration).

Change-Id: Ib7916c9664a0f72e40156a03bdfc79a4a6c24350
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>