Variables in makefile prerequisites - makefile

Question

Is it possible to use makefile variables in prerequisites? My example below is a little backward, but should demonstrate what I'm trying to achieve:
objects_subsystem1 := $(patsubst %.c,%.o,$(wildcard ../src/ss1/*.c))
objects_subsystem2 := $(patsubst %.c,%.o,$(wildcard ../src/ss2/*.c))
all : subsystem1.elf subsystem2.elf
%.elf : $(objects_%)
$(LD) $< -o $#
EDIT:
I'm using GNU Make 3.80 so unfortunately SECONDEXPANSION is not available!

Answer 1

Yes, if you're using GNUMake:
.SECONDEXPANSION:
%.elf : $$(objects_%)
$(LD) $< -o $#
EDIT:
As #bobbogo points out, there is a solution that does not require .SECONDEXPANSION. First, we spell out the rules:
objects_subsystem1 := $(patsubst %.c,%.o,$(wildcard ../src/ss1/*.c))
objects_subsystem2 := $(patsubst %.c,%.o,$(wildcard ../src/ss2/*.c))
objects_subsystem3 := $(patsubst %.c,%.o,$(wildcard ../src/ss3/*.c))
all : subsystem1.elf subsystem2.elf subsystem3.elf
subsystem1.elf : $(objects_subsystem1)
$(LD) $< -o $#
subsystem2.elf : $(objects_subsystem2)
$(LD) $< -o $#
subsystem3.elf : $(objects_subsystem3)
$(LD) $< -o $#
Then we rearrange things a little:
objects_subsystem1 := $(patsubst %.c,%.o,$(wildcard ../src/ss1/*.c))
all : subsystem1.elf
subsystem1.elf : $(objects_subsystem1)
objects_subsystem2 := $(patsubst %.c,%.o,$(wildcard ../src/ss2/*.c))
all : subsystem2.elf
subsystem2.elf : $(objects_subsystem2)
objects_subsystem3 := $(patsubst %.c,%.o,$(wildcard ../src/ss3/*.c))
all : subsystem3.elf
subsystem3.elf : $(objects_subsystem3)
%.elf :
$(LD) $< -o $#
Then we use the eval function:
define RULE_TEMPLATE
objects_subsystem$(1) := $(patsubst %.c,%.o,$(wildcard ../src/ss$(1)/*.c))
all : subsystem$(1).elf
subsystem$(1).elf : $$(objects_subsystem$(1))
endef
$(eval $(call RULE_TEMPLATE,1))
$(eval $(call RULE_TEMPLATE,2))
$(eval $(call RULE_TEMPLATE,3))
%.elf :
$(LD) $< -o $#
Then finish up with a loop (if it's worth it):
define RULE_TEMPLATE
objects_subsystem$(1) := $(patsubst %.c,%.o,$(wildcard ../src/ss$(1)/*.c))
all : subsystem$(1).elf
subsystem$(1).elf : $$(objects_subsystem$(1))
endef
SUBSYSTEMS := 1 2 3 # this can be made automatic...
$(foreach sys,$(SUBSYSTEMS),$(eval $(call RULE_TEMPLATE,$(sys))))
%.elf :
$(LD) $< -o $#