Use relative paths when invoking mifconv.
Use relative paths when invoking mifconv.

Some S60 SDK has a bug which make mifconv to crash with absolute paths.
Switched to use QMAKE_FILE_IN variable in mifconv.commands instead of
using join of ICON keywords. Joining is not needed anymore since ICON
keyword supports only one filename. Using input variable
(QMAKE_FILE_IN) also esnures that absolute path is converted to
relative before invoking mifconv.

Task-number:  QTBUG-4745
Reviewed-by: Miikka Heikkinen
(cherry picked from commit 745db3d1b9a149910dcaa3c65aabab020eefa027)

pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk