GNU make path substitution (directory flattening) - makefile

Question

I have a makefile that gives me the source files in a hierarchy.
SRCS := $(wildcard $(SRC_DIR)/*.c $(SRC_DIR)/*/*.c)
gives me
./src/main.c ./src/add/add.c ./src/sub/sub.c
I want to flatten the object files into a single "obj" directory.
Of course
OBJS := $(SRCS:$(SRC_DIR)/%.c=$(OBJ_DIR)/%.obj)
gives me
./obj/main.obj ./obj/add/add.obj ./obj/sub/sub.obj
instead of desired
./obj/main.obj ./obj/add.obj ./obj/sub.obj
Question: How do I get rid of additional source directory levels?
Never had to use complex substitution so far. My intuitive try with additional "/%" in pattern
# won't work as expected:
OBJS := $(SRCS:$(SRC_DIR)/%/%.c=$(OBJ_DIR)/%.obj)
produces no meaningful result (${OBJS} becomes same as ${SRCS}).
All examples I found so far only have single occurance of "%" in match pattern.
MAK_DIR := $(dir $(lastword $(MAKEFILE_LIST)))
SRC_DIR = $(MAK_DIR)./src
OBJ_DIR = $(MAK_DIR)./obj
# gives: ./src/main.c ./src/add/add.c ./src/sub/sub.c
SRCS := $(wildcard $(SRC_DIR)/*.c $(SRC_DIR)/*/*.c)
# gives: ./obj/main.obj ./obj/add/add.obj ./obj/sub/sub.obj
OBJS := $(SRCS:$(SRC_DIR)/%.c=$(OBJ_DIR)/%.obj)
.PHONY : all
all :
#echo $(SRCS)
#echo $(OBJS)

Answer 1

Just use the notdir function, and also patsubst, more powerful (and that I find easier to understand than the shorthand):
OBJS := $(patsubst %.c,$(OBJ_DIR)/%.obj,$(notdir $(SRCS)))
But it is not the whole story because later on you will probably want to do something like:
$(OBJ_DIR)/%.obj: $(SRC_DIR)/%.c
which will not work any more. Not mentioning the fact that you could have several source files with the same name in different directories. Assuming you do not have such names conflicts you can generate all your dependencies using foreach-eval-call:
MAK_DIR := $(dir $(lastword $(MAKEFILE_LIST)))
SRC_DIR := $(MAK_DIR)./src
OBJ_DIR := $(MAK_DIR)./obj
SRCS := $(shell find $(MAK_DIR) -type f -name '*.c')
OBJS := $(patsubst %.c,$(OBJ_DIR)/%.obj,$(notdir $(SRCS)))
.PHONY: objs
objs: $(OBJS)
# $(1): source file
define DEP_rule
$(1)-obj := $$(patsubst %.c,$$(OBJ_DIR)/%.obj,$$(notdir $(1)))
$(1)-dep := $$(patsubst %.c,$$(OBJ_DIR)/%.d,$$(notdir $(1)))
$$($(1)-obj): $(1) $$($(1)-dep)
endef
$(foreach src,$(SRCS),$(eval $(call DEP_rule,$(src))))
Just remember that the DEP_rule macro gets expanded twice, thus the $$.