Makefile for a Library with Separate Variants

The modifications for separate library variants are quite similar:

# Makefile for maintaining separate library variants.  

CFLAGS= -O 

SOURCES= main.c rest.c 
LIBRARY= lib.a 
LSOURCES= fnc.c 

OBJECTS= $(SOURCES:%.c=$(VARIANT)/%.o) 
VLIBRARY= $(LIBRARY:%.a=$(VARIANT)/%.a) 
LOBJECTS= $(LSOURCES:%.c=$(VARIANT)/%.o) 
VARIANT= .

program profile debug: $$(OBJECTS) $$(VLIBRARY) 
         	$(LINK.c) -o $(VARIANT)/$@ $< 

lib.a debug_dir/lib.a profile_dir/lib.a: $$(LOBJECTS) 
         	ar rv $@ $? 

$$(VLIBRARY)($$(VARIANT)%.o): $$(VARIANT)%.o 
        	@true 
profile := VARIANT = profile_dir
profile := CFLAGS = -O -pg 

debug := VARIANT = debug_dir
debug := CFLAGS = -g 

.KEEP_STATE:
profile_dir debug_dir:
        	test -d $@ || mkdir $@ 
$$(VARIANT)/%.o: %.c 
        	$(COMPILE.c) $< -o $@

While an interesting and useful compilation technique, this method for maintaining separate variants is a bit complicated. For the sake of clarity, it is omitted from subsequent examples.