How to use pkg-config in Make - c

Question

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.

Answer 1

There are two issues.
First, your CFLAGS line is wrong: you forgot to say gtk+-3.0 in the pkg-config part, so pkg-config will spit out an error instead:
CFLAGS=-g -Wall -Wextra $(pkg-config --cflags gtk+-3.0)
Second, and more important, $(...) is intercepted by make itself for variable substitution. In fact, you've seen this already:
SOURCES=$(wildcard *.c)
EXECUTABLES=$(patsubst %.c,%,$(SOURCES))
all: $(EXECUTABLES)
is all done by make.
There are two things you can do.
First, you can use `...` instead, which does the same thing ($(...) is newer shell syntax).
CFLAGS=-g -Wall -Wextra `pkg-config --cflags gtk+-3.0`
LDFLAGS=`pkg-config --libs gtk+-3.0`
Second, since you seem to be using GNU make, you can use the shell substitution command, which was shown in the answer Basile Starynkevitch linked above:
CFLAGS=-g -Wall -Wextra $(shell pkg-config --cflags gtk+-3.0)
LDFLAGS=$(shell pkg-config --libs gtk+-3.0)