Always insert space between "-prefix" and path.

Otherwise I get "-prefix/my/installation/path" which results in an error.

Change-Id: I087803988ae6db8857f607a05e8d9eef91b8d4bd
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>
Reviewed-by: Andreas Holzammer <andreas.holzammer@kdab.com>