Disable building json2cbor.c in tinycbor, and which doesn't build on macOS/BSDs when cjson is installed

[kkysen]

Fixes #353.

This unconditionally disables building tools/json2cbor/ in tinycbor, which isn't needed for our uses of tinycbor and is an optional dependency. Normally it is autodetected when cjson is found, and it still is, but now we append to the generated .config file to explicitly disable it.

On macOS/BSD, tools/json2cbor/json2cbor.c doesn't build because it defines _POSIX_C_SOURCE, which in BSD libc, prevents asprintf from being declared, which is used by tools/json2cbor/json2cbor.c. This can be fixed by removing the _POSIX_C_SOURCE define, but that'd have to be fixed in a patch or upstream in https://github.com/intel/tinycbor, which it probably should be, but the fix would take longer.

However, either way, we don't need json2cbor, so we can just always disable it, and it's better to be explicit about our dependencies than unknowingly having an optional dependency on cjson that affects the build.

[kkysen]

@gcxfd, can you see if this fixes your tinycbor issue when compiling from cargo install c2rust?

[kkysen]

It's fixed upstream, too, now: intel/tinycbor#221.

So we don't need this fix anymore (if we upgrade our tinycbor version), but I think it's still worth it to have, because otherwise we're pulling in an unneeded dependency, but only sometimes, and there could be some other future error there.

[kkysen]

intel/tinycbor#221 has now been tested on macOS by @boyland-pf and it works, so we just need to test this one now (with cjson installed globally) if anyone with a mac can:

cargo install c2rust --git https://github.com/immunant/c2rust --branch kkysen/tinycbor-disable-cjson

Once that's tested, we should be able to merge this I think.

[boyland-pf]

This also compiles on mac.

[fw-immunant]

Thanks for tracking down the issue here. Fix looks good.