Cmake: How to include dependecies properly - cmake

Question

In the old Makefile there is an include part:
SHELL=/bin/sh
CORE_SRC=\
./Core/allocator.cpp \
./Core/etc... \
CORE_OBJS=$(CORE_SRC:.cpp=.o)
INCLUDE=\
`pkg-config --cflags glib-2.0 libpng` \
`sdl-config --cflags` \
`freetype-config --cflags` \
`./python-config-linux.sh --cflags` \
-I./TopLayer -I./etc...
CC=g++-4.4
CFLAGS=-O3 -pipe -Wall -fPIC -D__STDC_CONSTANT_MACROS
CORE_LFLAGS=\
-fPIC \
-Wl,-rpath,./libs
CORE_LDLIBS=\
`pkg-config --libs glib-2.0 libpng` \
`sdl-config --libs` -lz -ljpeg \
`freetype-config --libs` \
`curl-config --libs` \
-L./$(LIBSDIR) \
-letc...
GAME_LFLAGS=\
-shared -pthread -fPIC \
-Wl,-rpath,../libs
GAME_LDLIBS=\
-lm \
`python-config-linux.sh --libs`
target_name: $(CORE_OBJS) $(CORE_NAME)
target_name: override CFLAGS += -DAV_OUTPUT
$(CORE_NAME): $(CORE_OBJS)
$(CC) $(CORE_LFLAGS) $(CORE_OBJS) $(CORE_LDLIBS) -o $#
.c.o:
$(CC) $(CFLAGS) $(INCLUDE) -c $< -o $*.o
.cpp.o:
$(CC) $(CFLAGS) $(INCLUDE) -c $< -o $#
The execution of those configs is:
$pkg-config --cflags glib-2.0 libpng
-> -I/usr/include/glib-2.0 -I/usr/lib/i386-linux-gnu/glib-2.0/include -I/usr/include/libpng12
$sdl-config --cflags
-> -I/usr/include/SDL -D_GNU_SOURCE=1 -D_REENTRANT
$freetype-config --cflags
-> -I/usr/include/freetype2
$./python-config-linux.sh --cflags
-> -I/python-2.7.10/include/python2.7 -I/python-2.7.10/include/python2.7 -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes
I don't understand and a bit confused about Cmake - should I use target_include_dirictories() or find_packages()?
Should target_include_dirictories() be used also with --cflags and libs like glib-2.0 libpng for pkg-config?
*Updated makefile. Removed unnecessary targets etc.

Answer 1

You will need both find_package() and target_link_libraries().
target_include_directories() is almost always obsolete when using libraries found through find_package. Docs.
Example
Example tested on macOS HighSierra with CMake 3.12.
project(Example)
cmake_minimum_required(VERSION 3.10)
find_package(PNG REQUIRED)
add_executable(prog main.c) # main.c from: http://zarb.org/~gc/html/libpng.html
target_link_libraries(prog PNG)
# On Ubuntu 14.04 + CMake 3.2 use the older syntax (FindPNG.cmake is too old) from the next line:
# target_link_libraries(prog ${PNG_LIBRARIES})