Add install rules

Otherwise d3dcompiler will not be installed using -prefix in configure

Change-Id: I3054969cc5d8b7c2378b12bbaef3dc4ca0194711
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
remove doubled specification

DLLDESTDIR is set in config.pri already, hence no need to set it twice

Change-Id: I84a5ecd0a3db41ff3920a295644e3d169deed264
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>