Configure: Set separate_debug_info for all modules

Configure: Set separate_debug_info for all modules

separate_debug_info was previously stored in .qmake.cache, and
therefore only affected qtbase. Store it as a QT_CONFIG setting instead,
and add it to CONFIG for all Qt plugins and modules.

Change-Id: I940c8cb30f1c42c85f4d0342e6a482f20677ff04
Reviewed-on: http://codereview.qt.nokia.com/2975
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Marius Storm-Olsen <marius.storm-olsen@nokia.com>