CMake convention is to use a project namespace, i.e. Foo::, for imported
targets. When multiple targets are imported from a project, this looks
like Foo::Bar1 Foo::Bar2, etc. This adds the nlohmann_json:: namespace to
the exported target names.
This also allows the generated project config files to be used from the
build directory instead of just the install directory.
The rewrite uses more cmake build-in automatisms and build-in generates
variables to allow better generic reuse.
* 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> ```
- define the project's version in the cmake listfile
- create a config and config version file for the cmake package
- install the json.hpp file and the package's targets, config and
config version files
