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`.
Merge branch 'relx-output-dir-fix' of git://github.com/jtmoulia/erlang.mk