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`.

plugins/relx.mk

@@ -16,8 +16,8 @@ RELX_URL ?= https://github.com/erlware/relx/releases/download/v1.0.2/relx
 RELX_OPTS ?=
 RELX_OUTPUT_DIR ?= _rel
 
-ifneq ($(firstword $(subst -o,,$(RELX_OPTS))),)
-	RELX_OUTPUT_DIR = $(firstword $(subst -o,,$(RELX_OPTS)))
+ifeq ($(firstword $(RELX_OPTS)),-o)
+	RELX_OUTPUT_DIR = $(word 2,$(RELX_OPTS))
 endif
 
 # Core targets.