Provide nlohmann/json dependency with multiple headers

[vid512]

meson.build of nlohmann/json contains additional dependency with multiple headers, instead of single one. This PR allows wrapDB users to use it.

Multiple headers are useful eg. if you want to forward-declare nlohmann::json object (see json_fwd.hpp).

[eli-schwartz]

IIRC nlohmann_json is never installed this way and the dependency lookup won't ever work for the general case. Not sure this is a good idea...

[vid512]

What do you mean "this way"? Via wrapdb? I don't follow.

I just tested it:

• installed dependency wrap with meson wrap install nlohmann_json,
• manually edited subprojects/nlohmann_json.wrap as in c1b781c, to simulate this PR
• included dependency('nlohmann_json_multiple_headers') in meson.build
• now cpp file with #include <nlohmann/json_fwd.hpp> and forward-declared nlohmann::json compiles just fine

[dcbaker]

Me either. I'm looking, and I would rather see if upstream would take an update to their Meson. I'm opening a PR right now for that purpose. (I use this project downstream, so I have some interest in making this better)

[dcbaker]

nlohmann/json#4452

[vid512]

OK I see my mistake now. For whatever reason I thought json_fwd.hpp is only available with multiple headers, I completely missed that json_fwd.hpp is in single_include as well. Yes, this is pointless. At least it prompted update upstream. :)

[eli-schwartz]

The other factor here is that the usual name is pkg-config --cflags nlohmann_json or for cmake, find_package(nlohmann_json).

Implementing a new dependency name would mean it only worked as a wrap, but not as a system package, which is a naming wart and probably a leaky internal implementation detail. And implementation details could probably be used by projects that really want them, via subproject().get_variable().

It would generally only matter for projects that don't care about the forward declarations, but rather want to overly specify whether they use amalgamated headers.