Add support for Qt resources in Subsurface

Signed-off-by: Thiago Macieira <thiago@macieira.org>
Don't hardcode the paths for mkdir: just get them from the target

The $(@D) (equivalent to $(dir $@)) tells us what the directory the
target is in. We could also have used the one for the source. They're
equivalent there.

Signed-off-by: Thiago Macieira <thiago@macieira.org>