Github issue:
https://github.com/nlohmann/json/issues/2073

nlohmann::json documents 2 way of depending on it using CMake
1) Copy-paste the project/source into your own project.
2) Install nlohman::json and then use find_package.

(1) pollutes your git repository, (2) requires everyone to install the
dependencies themselves.

Since 2018, CMake provide some kind of 'package manager' features using
[FetchContent](https://cmake.org/cmake/help/v3.17/module/FetchContent.html)
It gives the following:
~~~cmake
include(FetchContent)

FetchContent_Declare(json
  GIT_REPOSITORY https://github.com/nlohmann/json
  GIT_TAG v3.7.3)

FetchContent_GetProperties(json)
if(NOT json_POPULATED)
  FetchContent_Populate(json)
  add_subdirectory( ${json_SOURCE_DIR} ${json_BINARY_DIR} EXCLUDE_FROM_ALL)
endif()
~~~

Then declares the dependency in the target using it:
~~~cmake
target_link_library(my_project PRIVATE nlohmann_json::nlohmann_json
~~~

This patch updates the documentation and provides tests.

Fix returning reference to local temporary object

updated wandbox link

Add missing testcase about NaN in unit-constructor1.cpp

Fix bug in diff function

Fix GCC compiler warnings

Fix Clang compiler warnings

Update doctest to 2.3.7

make CMake's version config file architecture-independent

fix warnings in serializer.hpp for VS2019

Fix C26451 warnnings in to_chars.hpp

Add binary type support to all binary file formats, as well as an internally represented binary type

Add binary type support to all binary file formats, as well as an internally represented binary type

catch exceptions for json_pointer : ..../+99

Templatize basic_json constructor from json_ref

Replace deprecated std::is_pod

Modify the document about operator==

Added wsjcpp.yml

Fixes #1971 (memory leak in basic_json::push_back)

appveyor.yml: Compile and test with latest version for __cplusplus ma…