fix rcc & uic .depend_commands

unlike the .command, the .depend_command is not executed by make via its
chosen shell, but qmake itself via the system's native shell.
consequently, it needs different path separators and no make-escaping.

Task-number: QTBUG-31289
Change-Id: I480f815753632db6e8d4725f463f8a1fc59680a6
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>