handle configure's -I flags like configure.exe does

it's more correct that way.

Task-number: QTBUG-35588
Change-Id: I8dc55d6cd7dc08ebafd458b3df54fd69c3d91849
Reviewed-by: William Gallafent <william@gallaf.net>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>