REFACTOR: rewrite CMakeLists.txt for better inlcude and reuse

[dan-42]

The rewrite uses more cmake build-in functions and build-in automaticly generated
variables to allow better generic reuse.
This is based on the cmake example project package-example

• cmake files are installed to
  <install_prefix>/lib/cmake/nlohmann_json/ for best support on
  most systems
• include path is set to include for usage as #include <nlohmann/json.hpp>

Motivation

The motivation is the reintegration of nlohmann_json into the cmake package-manager hunter
and to avoid possible naming collissions with other packages( e.g. jsoncpp, etc)

compile install and run with unit tests

dan@kay ~/git/json/build (git)-[change_installed_cmake_include_path] % rm -rf ./* && rm -rf ../test_install/* && cmake ..  -DCMAKE_INSTALL_PREFIX=../test_install -DJSON_BuildTests=ON && make -j8  && make install
zsh: sure you want to delete all 8 files in /home/dan/git/json/build/. [yn]? y
zsh: sure you want to delete all 2 files in /home/dan/git/json/build/../test_install [yn]? y
-- The C compiler identification is GNU 7.1.1
-- The CXX compiler identification is GNU 7.1.1
[....]
-- Configuring done
-- Generating done
-- Build files have been written to: /home/dan/git/json/build
Scanning dependencies of target catch_main
[  2%] Building CXX object test/CMakeFiles/catch_main.dir/src/unit.cpp.o
[...]
[100%] Built target json_unit
make -j8  157.98s user 5.96s system 580% cpu 28.240 total
[  2%] Built target catch_main
[100%] Built target json_unit
Install the project...
-- Install configuration: ""
-- Installing: /home/dan/git/json/test_install/include/nlohmann
-- Installing: /home/dan/git/json/test_install/include/nlohmann/json.hpp
-- Installing: /home/dan/git/json/test_install/lib/cmake/nlohmann_json/nlohmann_jsonConfig.cmake
-- Installing: /home/dan/git/json/test_install/lib/cmake/nlohmann_json/nlohmann_jsonConfigVersion.cmake
-- Installing: /home/dan/git/json/test_install/lib/cmake/nlohmann_json/nlohmann_jsonTargets.cmake
dan@kay ~/git/json/build (git)-[change_installed_cmake_include_path] % ctest -j2
Test project /home/dan/git/json/build
    Start 1: json_unit_default
    Start 2: json_unit_all
1/2 Test #1: json_unit_default ................   Passed   12.12 sec
2/2 Test #2: json_unit_all ....................   Passed  223.00 sec

100% tests passed, 0 tests failed out of 2

Total Test time (real) = 223.01 sec
ctest -j2  234.59s user 0.52s system 105% cpu 3:43.01 total

Usage on an external project after installation

CMakeLists.txt

cmake_minimum_required (VERSION 3.0)
project(test_json)
find_package(nlohmann_json CONFIG REQUIRED PATHS /home/dan/git/json/test_install/)
add_executable(main main.cpp)
target_link_libraries(main nlohmann_json)

main.cpp

#include <iostream>
#include <nlohmann/json.hpp>
int main (int argc, char** argv) {
  nlohmann::json j2 = {
    {"pi", 3.141},    
  };
  std::cout << j2 << std::endl;
  return 0;
}

I'm looking forward on comments and thoughts about the changes.
Many thanks in advance

[mention-bot]

@dan-42, thanks for your PR! By analyzing the history of the files in this pull request, we identified @robertmrk, @nlohmann and @ChrisKitching to be potential reviewers.

[coveralls]

Coverage remained the same at 100.0% when pulling f434942 on dan-42:change_installed_cmake_include_path into c819a2d on nlohmann:develop.

[nlohmann]

Just out of curiosity: why did you remove the LANGUAGES part?

[dan-42]

because it is enabled by default, so no need for that. see https://cmake.org/cmake/help/v3.0/command/project.html

[nlohmann]

Thanks for checking back! Just out of coincidence: would it help to explicitly enable C++, because we do not use C at all?

[nlohmann]

Thanks a lot!

[dan-42]

awesome thank you