Dynamically generating a list of targets - makefile

Question

If a file exists, I want to add a target to build. If the file does not exist, I want the target to be skipped.
an example:
FILENAME = f
TARGETS := normal
ifneq($(shell stat test_$(FILENAME).c), "")
TARGETS += test
endif
all: $(TARGETS)
normal:
#echo normal
test:
#echo test
I'm not sure the $(shell stat ...) part even works, but the bigger problem is that make with any file test_f.c in the current folder gives:
Makefile:4: *** multiple target patterns. Stop.
Removing the ifneq ... endif block makes the target normal. How can I only run the target test if test_f.c exists?

Answer 1

What you can do is generate a string variable (let's call it OPTIONAL) such that when 'test_f.c' exists, OPTIONAL=test; otherwise, OPTIONAL=_nothing_. And then add OPTIONAL as a prerequisite of all. e.g.:
FILENAME = f
TARGETS = normal
OPTIONAL = $(if $(wildcard test_f.c), test, )
all: $(TARGETS) $(OPTIONAL)
normal:
#echo normal
test:
#echo test

Answer 2

You can also iterate over targets with for loop
.PHONY: all
RECIPES = one
all: RECIPES += $(if $(wildcard test_f.c), two, )
all:
for RECIPE in ${RECIPES} ; do \
$(MAKE) $${RECIPE} ; \
done
one:
$(warning "One")
two:
$(warning "Two")
> make
for RECIPE in one ; do \
/Applications/Xcode.app/Contents/Developer/usr/bin/make ${RECIPE} ; \
done
makefile:11: "One"
make[1]: `one' is up to date.
> touch test_f.c
> make
for RECIPE in one two ; do \
/Applications/Xcode.app/Contents/Developer/usr/bin/make ${RECIPE} ; \
done
makefile:11: "One"
make[1]: `one' is up to date.
makefile:14: "Two"
make[1]: `two' is up to date.