out-of-source-tree build :)

.SECONDEXPANSION:
.PHONY: all clean

OUT = out
SRC_all = 1.c in/2.cc
OBJ_all = $(SRC_all:%=$(OUT)/%.o)
OBJ += $(OBJ_all)

all: $(OBJ_all)

clean:
    rm -rf $(OUT)

$(OBJ): $(OUT)/%.o: % | $$(@D)/

%/:
    mkdir -p $@
%.c.o:
    @touch $@
    @echo gcc -o $@ $+
%.cc.o:
    @touch $@
    @echo g++ -o $@ $+

### krd@ntbk:~/tmp/test$ find . -printf '%P\n' | xargs -r ls -lhd
### -rw-r--r-- 1 krd krd   0 Feb 11 12:33 1.c
### -rw-rw-rw- 1 krd krd 331 Feb 11 17:14 Makefile
### drwxr-xr-x 2 krd krd  60 Feb 11 17:14 in
### -rw-r--r-- 1 krd krd   0 Feb 11 12:33 in/2.cc
###
### krd@ntbk:~/tmp/test$ make
### mkdir -p out/
### gcc -o out/1.c.o 1.c
### mkdir -p out/in/
### g++ -o out/in/2.cc.o in/2.cc
###
### krd@ntbk:~/tmp/test$ find . -printf '%P\n' | xargs -r ls -lhd
### -rw-r--r-- 1 krd krd   0 Feb 11 12:33 1.c
### -rw-rw-rw- 1 krd krd 331 Feb 11 17:14 Makefile
### drwxr-xr-x 2 krd krd  60 Feb 11 17:14 in
### -rw-r--r-- 1 krd krd   0 Feb 11 12:33 in/2.cc
### drwxr-xr-x 3 krd krd  80 Feb 11 17:15 out
### -rw-r--r-- 1 krd krd   0 Feb 11 17:15 out/1.c.o
### drwxr-xr-x 2 krd krd  60 Feb 11 17:15 out/in
### -rw-r--r-- 1 krd krd   0 Feb 11 17:15 out/in/2.cc.o