Use the INSTALL_PREFIX cmake option.

Call this little script from your Makefile:

git clone https://github.com/liballeg/allegro5.git
mkdir build-allegro5
cd build-allegro5
cmake -D CMAKE_INSTALL_PREFIX=install -D WANT_MONOLITH=1 -D WANT_DOCS=0 -D WANT_EXAMPLES=0 -D WANT_DEMO=0 -D SHARED=0 ../allegro5
make -j4
make install

First line of course only if you don't distribute Allegro5 yourself. You can leave out the WANT_MONOLITH and SHARED options if you prefer something else.

Now this will work in your game's main Makefile (assuming it's in the same directory you ran the script from):

PKG_CONFIG_PATH=build-allegro5/install/lib/pkgconfig pkg-config allegro_monolith-static-5 --libs --cflags

For me this outputs:

-I/home/elias/prog/allegro/tmp/build-allegro5/install/include -L/home/elias/prog/allegro/tmp/build-allegro5/install/lib -lallegro_monolith-static -lm -lpthread -lSM -lICE -lX11 -lXext -lXcursor -lXi -lXinerama -lXrandr -lGLU -lGL -lSM -lICE -lX11 -lXext -lpng -lz -ljpeg -lpulse-simple -lpulse -lasound -lopenal -lFLAC -logg -ldumb -lvorbisfile -lvorbis -logg -lfreetype -lz -lphysfs -lgtk-x11-2.0 -lgdk-x11-2.0 -lpangocairo-1.0 -latk-1.0 -lcairo -lgdk_pixbuf-2.0 -lgio-2.0 -lpangoft2-1.0 -lpango-1.0 -lgobject-2.0 -lglib-2.0 -lfontconfig -lfreetype -lgthread-2.0 -lglib-2.0 -ltheoradec -logg -lvorbisfile -lvorbis -logg 

I would have left it as an external script and just call from the Makefile - this is the best I got it to work from within a Makefile:

ALLEGRO_GIT = https://github.com/liballeg/allegro5.git
ALLEGRO_OPTIONS = `pkg-config allegro_monolith-static-5 --libs --cflags --static`
export PKG_CONFIG_PATH = build-allegro5/install/tlib/pkgconfig

TEST: ALLEGRO
	echo $(ALLEGRO_OPTIONS)

ALLEGRO: allegro5
	cd allegro5; git pull
	mkdir -p build-allegro5
	cd build-allegro5;\
	cmake -D CMAKE_INSTALL_PREFIX=install -D WANT_MONOLITH=1 -D WANT_DOCS=0 -D WANT_EXAMPLES=0 -D WANT_DEMO=0 -D SHARED=0 ../allegro5;\
	make -j4;\
	make install

allegro5:
	git clone ${ALLEGRO_GIT}

As for the theoradec issue, I don't know. What is the exact command that's executed and causes the error, and what is the full error message?

Hm, try passing -D WANT_VIDEO=0 to cmake and see if the error disappears.

I see another problem, do this:

ALLEGRO_CFLAGS = `pkg-config allegro_monolith-static-5 --cflags --static`
ALLEGRO_LIBS = `pkg-config allegro_monolith-static-5 --libs --static`

And then later:

$(EXC): $(OBJECT)
    gcc $(EFLAGS) -o $(EXC) $(OBJECT) $(ALLEGRO_LIBS)

$(OBJFOLD)/%.o: %.c
    gcc $(CFLAGS) $(ALLEGRO_CFLAGS) $< -o $@

Basically you need different flags for compiling and linking.

Inside the makefile, use the export command to set the environment variable:

export PKG_CONFIG_PATH = build-allegro5/install/lib/pkgconfig

What does your complete script look like now?

Are you certain your export commands work? For me it only works if I have the export at the top of the Makefile, like in my version.

Other than that I have no idea - can you show the complete make output? Maybe there's some other clues there...

That means pkg-config did not work, try with the export like I have it.

[edit:] Oh, I see, you didn't get that before... should not matter then. (But in my version, you need to put it at the top, not inside any rule.) Anyway, as long as that error isn't there either version apparently works.

[edit2:] Also, the parameter order is wrong, pkg-config --libs has to come after the sources.

Hm, there's no reason it shouldn't work, it does here. Try this, just typing in a console:

export PKG_CONFIG_PATH="build-allegro5/install/lib/pkgconfig"
gcc Main.c -o obj/Main.o `pkg-config allegro_monolith-static-5 --cflags --static`

Does it also given an error about not finding Allegro functions?

Oh, true, try adding -DALLEGRO_STATICLINK to the options.