When adding this library as embedded library with private
"target link", e.g. only used inside private source files, the
library does not need to be installed when the main project gets
installed.
This adds an additional option `JSON_Install` similar to the
test-build control switch in order to skip installing headers and
CMake config files if requested.
Avoids using
```cmake
add_subdirectory(path/to/nlohmann_json EXCLUDE_FROM_ALL)
```
which has further side-effects:
https://cmake.org/cmake/help/v3.0/command/add_subdirectory.html
I contributed a package to the Spack package manager.
Spack is a flexible package manager that supports multiple versions,
configurations, platforms, and compilers. It is popular in
high-performance computing.
Axel Huebl