CMake-3.22 on Ubuntu-22.04 fails to fetch nlohmann library

[mikucionisaau]

The issues are in cmake/nlohmann.cmake:

1. FetchContent_Declare statement fails with "At least one entry of URL is a path (invalid in a list)" which indicates that cmake is unable to parse the URL parameter correctly (probably cmake bug), I had to swap the arguments like this:

FetchContent_Declare(json
  DOWNLOAD_EXTRACT_TIMESTAMP ON
  URL https://github.com/nlohmann/json/releases/download/v3.11.2/json.tar.xz
)

2. add_subdirectory fails with "add_subdirectory called with incorrect number of arguments". The issue is the extra SYSTEM argument which was introduced much later than cmake-3.22. Meanwhile the root CMakeLists.txt declares to use cmake-3.15, so this is not correct. Suggested fix:

   add_subdirectory(${json_SOURCE_DIR} ${json_BINARY_DIR} EXCLUDE_FROM_ALL)

(EXCLUDE_FROM_ALL excludes nlohmann targets from the main project targets minimizing development clutter)

[mikucionisaau]

fixed in #9