Debugging helpers: Fix building

- Report errors correctly.
- Do not pass empty target argument to qmake
- Give 'make' a longer timeout and change timeout checking to
  trigger after last output line was obtained from process
- Make build log dialog non-modal and ensure it pops up on failure
  even if a different Qt version was selected in-between
- Disable Rebuild-button while build is running.

Reviewed-by: Kai Köhne <kai.koehne@nokia.com>
Reviewed-by: Lasse Holmstedt <lasse.holmstedt@nokia.com>