settings.mk : subst construct

Hello

I am on master branch, and in  
http://opengrok.libreoffice.org/xref/core/solenv/inc/settings.mk#544

I see that :
LOCAL_COMMON_OUT:=$(subst,$(OUTPATH),$(COMMON_OUTDIR) $(OUT))

This construct of subst is quite unusual, and a sample Makefile with
OUT* := /cygdrive/d/src/libo/wntmsci12.pro
OUTPATH := wntmsci12
COMMON_OUTDIR := $(OUTPATH)
LOCAL_COMMON_OUT:=$(subst,$(OUTPATH),$(COMMON_OUTDIR) $(OUT))

.PHONY : test

test :

        @echo PWD: $(PWD)
        @echo $(LOCAL_COMMON_OUT)

Leads to no output for the 2nd echo.
Is the construct legal or is there an issue here ?
If it is legal, I'd love to have an explanation, because I had no great  
success when googling.

Thanks for the input

--
Mat M