Add support to generate compilation database

[neilweidinger]

Description

Generates a compile_commands.json compilation database, used for semantic auto-complete in text editors using the clangd language server. Can be used for text editors that support the language server protocol, like VSCode and Vim :))

https://sarcasm.github.io/notes/dev/compilation-database.html

[neilweidinger]

f, travis is failing because -MJ is only supported by clang, fixing now

[mkristien]

I do not have any major issues with this. Thanks for keeping the change set small.

A small suggestion:
Language server indexing is unrelated to the compilation cycle. Your implementation, on the other hand, only generates the indexing database compile_commands.json when you compile the code base. Until then, your editor would not make use of this. Furthermore, the database is destroyed when you clean the object files. Note, clean does not even remove the linter cache.

You could instead have a separate target (e.g. compiledb) that would create compile_commands.json if it does not already exist. You can run this once, independently from the edit->compile->decipher_error_log cycle. I use ccls for this. This has a simpler syntax that applies globally for all cpp files, which is an advantage, as the compilation database does not need to be recreated when adding new files.

[mkristien]

Are you not interested in extending this functionality to test files as well?

[mkristien]

Could you clean the temporary .json files once you do not need them?

[neilweidinger]

Sure, this should be possible, but I'm not quite sure why this would be beneficial? The .json files are in bin anyways, would there be a difference?

[neilweidinger]

Thanks for taking a look Martin! That's definitely a good point that language server indexing is unrelated to the compilation cycle.

My original rationale for putting it together during compilation was because it was just easy to pass the -MJ flag in our makefile where it handles compiling the files, and now that you point it out, I'm actually not sure why I made it so that the database gets deleted after running clean haha. I totally forgot about the test files, I'll make sure to include them.

Having the database as a separate target seems like a much better idea, I'll give it a shot at implementing it this way.