Fix usage of '-separate-debug-info' and '-force-debug-info'

  - this change does not apply to Qt 4.8
  - it was removed from -developer-build builds

Change-Id: I5f0b3247b539ee9815b5234e05a129060704760e
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Simo Fält <simo.falt@digia.com>