In follow up from the conversation in #1269, this adds the
`-Wno-deprecate-declarations` flag to unit tests to allow them to
test deprecated APIs without all the noisy compiler warnings.
This also refactors the setting of build properties for test targets
to use `target_<...>` commands instead of `set_target_properties()`.
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.
Chuck Atkins