fixed setting `RELX_OUTPUT_DIR` from `RELX_OPTS`
fixed setting `RELX_OUTPUT_DIR` from `RELX_OPTS`

This fix is imperfect: it only works if `-o $DIR` are the first args
in `RELX_OPTS`. It does not implicitly set the `RELX_OUTPUT_DIR` when
there is an arg other than `-o`, e.g. `--dev-mode`, leading
`RELX_OPTS`.

bsmr-erlang/erlang.mk,ninenines/erlang.mk,michaelnisi/erlang.mk,hairyhum/erlang.mk,jj1bdx/erlang.mk,yannayl/erlang.mk,nevar/erlang.mk,efcasado/erlang.mk,crownedgrouse/erlang.mk,dch/erlang.mk,KrzysiekJ/erlang.mk,layerhq/erlang.mk,a12n/erlang.mk,rabbitmq/erlang.mk,ingwinlu/erlang.mk