no subject

$ cat Makefile 
UNIQUE="an_unique_string_which_never_meets_in_filenames"

A1 = $(shell ls dir/*spaces | sed 's| |\\ |g')
A2 = $(subst $(UNIQUE),\ ,$(foreach filename,$(subst \ ,$(UNIQUE),$(A1)),$(filename).txt))

all: $(A2)

$(A2):
        echo test > "$@"

UNIQUE выбрать по вкусу.