Compiling and linking subproject library with CMake - compilation

Question

I have 2 projects (prj1 and prj2). One (prj2) depends on the other (prj1) that is a static library. I arrive to compile them separately with CMake.
But I needed to integrate one (prj1) to the other one (prj2). So I would like CMake to compile the static library (prj1) before the other (prj2) and then link the static library. I tried things, but id did not work.
prj1 "core" is https://gitlab.com/RyDroid/PlanetWars2dRT-core
prj2 "SDL2" is https://gitlab.com/RyDroid/PlanetWars2dRT-SDL2/ (see branch adding-core)
In prj2, externals/core is a git submodule (for non git users, you can see this directory as a copy-paste of prj1).
I tried (without success) this CMakeLists.txt in prj2 "SDL2":
cmake_minimum_required(VERSION 2.8)
if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU" OR
"${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
set(CMAKE_CXX_FLAGS "-Wall -Wextra -Wformat=2 -Wpedantic -D_FORTIFY_SOURCE=2")
set(CMAKE_CXX_FLAGS_DEBUG "-O0 -g")
elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
# TODO
endif()
set(CMAKE_CXX_FLAGS "-std=c++11 ${CMAKE_CXX_FLAGS}")
# if (CMAKE_VERSION VERSION_LESS "3.1")
# if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" OR
# CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
# set (CMAKE_CXX_FLAGS "-std=c++11 ${CMAKE_CXX_FLAGS}")
# endif()
# else()
# set(CMAKE_CXX_STANDARD 11)
# endif()
project(PlanetWars2dRT-SDL2)
# Version number
set(VERSION_MAJOR "0")
set(VERSION_MINOR "0")
set(VERSION_MICRO "0")
# Configure a header file to pass some of the CMake settings
# to the source code.
configure_file (
"src/compilation_config.h.in"
"${PROJECT_BINARY_DIR}/compilation_config.h"
)
# Add the binary tree to the search path for include files
# so that we will find compilation_config.h
include_directories("${PROJECT_BINARY_DIR}")
set(LIBRARY_OUTPUT_PATH lib/${CMAKE_BUILD_TYPE})
set(EXECUTABLE_OUTPUT_PATH bin/${CMAKE_BUILD_TYPE})
include_directories(src/)
file(
GLOB_RECURSE
source_files
src/*
)
add_executable(planet-wars-2d-rt-sdl2 ${source_files})
#include(ExternalProject)
#ExternalProject_Add(PlanetWars2dRT PREFIX externals/core/)
include_directories(externals/core/src/utils/ externals/core/src/specific/)
add_subdirectory(externals/core/)
#find_package(PlanetWars2dRT-core REQUIRED)
include(FindPkgConfig)
pkg_search_module(SDL2 REQUIRED sdl2)
pkg_search_module(SDL2GFX REQUIRED SDL2_gfx)
include_directories(${SDL2_INCLUDE_DIRS} ${SDL2IMAGE_INCLUDE_DIRS})
target_link_libraries(
planet-wars-2d-rt-sdl2
planet-wars-2d-rt-core
${SDL2_LIBRARIES} ${SDL2GFX_LIBRARIES}
)
That is a simplified version of the tree of prj2:
.
├── build (with CMake stuff generated with "cmake ..")
├── CMakeLists.txt
├── externals
│   └── core
│ ├── build (with CMake stuff generated with "cmake ..")
│   ├── CMakeLists.txt
│   ├── makefile
│   └── src
├── makefile
├── README.md
└── src
    ├── compilation_config.h.in
   └── planet-wars-2d-rt-sdl2.cpp
How can I compile the library of prj1 "core" in prj2 "SDL2" with CMake, and then link the library of prj1 with prj2 (again with CMake)?
If your solution does not work with a non GNU/Linux OS, it is not a big problem. Note: my PC is running on Debian GNU/Linux 8 "Jessie".
Regards.

Answer 1

I cannot access your repo https://gitlab.com/RyDroid/PlanetWars2dRT-SDL2/, perhaps the servers are down for maintenance. If I understand your problem correctly, you may create a structure like that:
root/
CMakeLists.txt // 1
src/
CMakeLists.txt // 2
main.cpp
SDL2/
some sources
CMakeLists.txt // 3
core/
some sources
CMakeLists.txt // 4
In the CMakeLists.txt [1] you should declare your project name, required packages, common flags, include paths etc.:
cmake_minimum_required( VERSION 2.x )
project(name)
include_directories(inc inc/core inc/SDL2 inc/SthElse)
add_subdirectory(src)
In CMakeLists.txt [2] for subdir src you should declare your main executable and also add subdirs with your prj1 and prj2
add_subdirectory(core)
add_subdirectory(SDL2)
add_executable(main main.cpp)
target_link_libraries(main core SDL2 SomeOtherLib)
Finally, in your CMakeLists.txt [3] && [4] in your lib dirs you should declare static libraries:
add_library(core STATIC ${YourSourceFiles})
This approach always worked for me. If you used to compile and run "core" and "SDL2" as standalone binaries, perhaps you will have to reorganize them a little bit.

Answer 2

Both project have a file "compilation_config.h". At compilation time, the wrong was taken so compilation failed, because the code used was #include "compilation_config.h" that was ambigous.
So I created a "include/project-name" dir in both project and I changed the include path : "prj1/compilation_config.h" or "prj2/compilation_config.h". Thanks to that there was no ambiguity anymore, so it works now!