Fix build errors on macOS

[stognini]

This PR fixes two build errors on macOS:

test/testdetail/TestMacrosImpl.cc:19:14: fatal error: 'nlohmann/json.hpp' file not found
#    include <nlohmann/json.hpp>
             ^~~~~~~~~~~~~~~~~~~

app/celer-g4/TimerOutput.hh:34:29: error: no template named 'unordered_map' in namespace 'std'
    using MapStrReal = std::unordered_map<std::string, real_type>;
                       ~~~~~^

[sethrj]

• The include_directories isn't necessary because the modern cmake targets propagate that information
• This should be PRIVATE not PUBLIC because it's only used in a .cc file
• if ( -> if(
• Our top-level cmakelists defines nlohmann_json_LIBRARIES so that it's empty if json isn't available, so that makes it easier to add for the optional cases
• It turns out I'm not directly using JSON anyway for now, so we could've just deleted the include 😅

[stognini]

Wait, so target_link_libraries(...) sets the include dirs as well?

[sethrj]

Exactly. Modern cmake targets propagate build flags, library flags, include directories, etc.