Add MAIN_PROJECT check for test and install options

[globberwops]

This resolves #2513 by checking if this is the main project in a CMake project tree.
JSON_BuildTests and JSON_Install will only be set to ON if this is the main project.
CTest will only be included, if JSON_BuildTests is set to ON.
BUILD_TESTING will therefore only be set, if this is the main project.

Pull request checklist

Read the Contribution Guidelines for detailed information.

• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header file single_include/nlohmann/json.hpp. The whole process is described here.

[coveralls]

Coverage remained the same at 100.0% when pulling ea759d0 on globberwops:develop into d8d8cbf on nlohmann:develop.

[hac01]

Can you teach me c++

[YarikTH]

For reference, here is the same purpose code from {fmt}

# Determine if fmt is built as a subproject (using add_subdirectory)
# or if it is the master project.
set(MASTER_PROJECT OFF)
if (CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR)
  set(MASTER_PROJECT ON)
  message(STATUS "CMake version: ${CMAKE_VERSION}")
endif ()
...
...
...
# Options that control generation of various targets.
option(FMT_DOC "Generate the doc target." ${MASTER_PROJECT})
option(FMT_INSTALL "Generate the install target." ${MASTER_PROJECT})
option(FMT_TEST "Generate the test target." ${MASTER_PROJECT})
option(FMT_FUZZ "Generate the fuzz target." OFF)
option(FMT_CUDA_TEST "Generate the cuda-test target." OFF)
option(FMT_OS "Include core requiring OS (Windows/Posix) " ON)

So it might be a good idea at least copy explanatory comment and maybe use CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_SOURCE_DIR check instead of CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR.

[nlohmann]

A last change request, otherwise looks good to me.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks!

[globberwops]

@nlohmann Thank you 👍🏻