A Makefile with Multiple Executables

For this particular case, where each executable has a single source file with .c extension, all you need is a one line Makefile:

all: ex1 ex3

The built-in default rules for make then work already:

$ make
cc -O2 -pipe   ex1.c  -o ex1
cc -O2 -pipe   ex3.c  -o ex3

Behind the scene, make is using the POSIXly mandated built-in single suffix rule

.c:
    $(CC) $(CFLAGS) $(LDFLAGS) -o $@ $<

Vary the command to your liking with make CC=gcc CFLAGS=-O2 LDFLAGS=-s and similar.

Trivia of the day: in fact, if you are willing to name the targets when invoking make, you can use an empty or even run without any Makefile:

$ make -f /dev/null CC=gcc CFLAGS=-O2 LDFLAGS=-s ex1 ex3
gcc -O2 -s ex1.c  -o ex1
gcc -O2 -s ex3.c  -o ex3
$ rm -f Makefile ex1 ex3
$ make CC=gcc CFLAGS=-O2 LDFLAGS=-s ex1 ex3
gcc -O2 -s ex1.c  -o ex1
gcc -O2 -s ex3.c  -o ex3

Make magic!

As a rule of thumb, don't reinvent the wheel (or rules), use the rules that are already there. It simplifies your and make's life a lot. This makes for small and sexy makefiles to impress the ladies with :-)

Some suggestions (assuming you use GNU make, not something else)

First, run once make -p, you'll understand what builtin rules make is knowing. Look in particular for COMPILE.c and LINK.c

Then, I suggest

 CFLAGS= -g -Wall -I.

^{(because you really want -g for debugging, and -Wall to get most warnings)}

And you probably don't need

$(EXECUTABLE): $(OBJ)
    gcc -o $@ $^ $(CFLAGS)

However, I suggest adding before most other rules

.PHONY: all clean

all: $(EXECUTABLES)

Actually, I would code your Makefile (for GNU make!) as follow

# file Makefile
CC= gcc
RM= rm -vf
CFLAGS= -Wall -g
CPPFLAGS= -I.
SRCFILES= ex1.c ex2.c ## or perhaps $(wildcard *.c)
OBJFILES= $(patsubst %.c, %.o, $(SRCFILES))
PROGFILES= $(patsubst %.c, %, $(SRCFILES))

.PHONY: all clean

all: $(PROGFILES)
clean:
     $(RM) $(OBJFILES) $(PROGFILES) *~
## eof Makefile

Remember that tab is a significant character in Makefile-s (action part of rules). In this answer, lines starting with four spaces at least should really start with a tab character.

Once everything is debugged consider running make clean to clean everything, and then make -j CFLAGS=-O2 all to compile in parallel everything with optimizations.

At last, I recommend using remake and running remake -x to debug complex Makefile-s

Of course, I'm supposing that your directory has only single-file programs.

BTW, there are other build automation tools. Perhaps you might consider using omake or ninja. For building large programs (millions of source code lines) consider also automake, ccache, cmake, icecream. In some cases, consider generating some C code with GPP, GNU bison, SWIG, etc... or using your own Python or Guile script (or C meta-program). See also this draft report.

^{Don't forget to use a version control system like git for your source files. It is also time to learn such a tool.}