Allow letting qmake do the link step

This adds a let_qmake_do_the_linking target variable to tell ninja
not to link but rather dump the linking information from gyp in a
file that we'll parse with qmake.

Change-Id: Icfaadae7ce4e9895f76b5fe06e05124e9b980e8b
Reviewed-by: Zoltan Arvai <zarvai@inf.u-szeged.hu>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>