Add notice about deprecation of the makefile (and fix it)

Makefile

@@ -3,19 +3,21 @@ BIN = /usr/bin
 INCLUDES = -I.
 PALETTE_DIR=\"$(PWD)/palettes\"
 CFLAGS = -O3 -Wall -Wextra -Wno-missing-field-initializers -DPALETTE_DIR=$(PALETTE_DIR) $(INCLUDES)
-OBJS = src/main.o src/image.o src/dsp.o src/filter.o src/reg.o src/pngio.o src/median.o src/color.o
+OBJS = src/main.o src/image.o src/dsp.o src/filter.o src/libs/reg.o src/pngio.o src/libs/median.o src/color.o src/libs/argparse.o
 
 aptdec:	$(OBJS)
 	$(CC) -o $@ $(OBJS) -lm -lsndfile -lpng
+	echo "Using the Makefile with GNU automake is now deprecated and will be removed soon, please use CMake instead."
 
-reg.o:    src/reg.c
-color.o:  src/color.c
-main.o:   src/main.c
-media.o:  src/median.c
-dsp.o:    src/dsp.c
-filter.o: src/filter.c
-image.o:  src/image.c
-pngio.o:  src/pngio.c
+src/libs/reg.o: src/libs/reg.c
+src/libs/argparse.o: src/libs/argparse.c
+src/color.o:    src/color.c
+src/main.o:     src/main.c
+src/libs/median.o: src/libs/median.c
+src/dsp.o:      src/dsp.c
+src/filter.o:   src/filter.c
+src/image.o:    src/image.c
+src/pngio.o:    src/pngio.c
 
 clean:
 	rm -f src/*.o aptdec

README.md

@@ -27,9 +27,13 @@ The input audio format can be anything supported by `libsndfile` (although only
 ```sh
 sudo apt install git gcc libsndfile-dev libpng-dev
 git clone https://github.com/Xerbo/aptdec.git && cd aptdec
+mkdir build && cd build
+cmake -DCMAKE_BUILD_TYPE=Release
 make
 ```
 
+Using the Makefile with GNU automake is now deprecated and will be removed soon.
+
 ## Examples
 
 To create an image from `gqrx_20200527_115730_137914960.wav` (output filename will be `gqrx_20200527_115730_137914960-r.png`)