How to use pkg-config in Make

I want to compile the simplest GTK program. I can compile it using the command line:

gcc $(pkg-config --cflags --libs gtk+-3.0)  main.c -o main.o

However, if I use Make it doesnt work:

CFLAGS=-g -Wall -Wextra $(pkg-config --cflags)
LDFLAGS=$(pkg-config --libs gtk+-3.0)
CC=gcc

SOURCES=$(wildcard *.c)
EXECUTABLES=$(patsubst %.c,%,$(SOURCES))

all: $(EXECUTABLES)

It tells me this:

gcc -g -Wall -Wextra    -c -o main.o main.c
main.c:1:21: fatal error: gtk/gtk.h: No such file or directory
 #include <gtk/gtk.h>
                     ^
compilation terminated.
<builtin>: recipe for target 'main.o' failed
make: *** [main.o] Error 1

Where do I stick $(pkg-config --cflags --libs gtk+-3.0) in the Makefile to make it compile?

Thanks very much in advance for your kind help.