Create compile-commands.json with Make

[andreabedini]

Motivation

The language server clangd needs to know the build flags which it reads from a file compile_commands.json ("JSON Compilation Database").

The contributing section of the manual suggests to create this file with

make clean && bear -- make -j$NIX_BUILD_CORES default check install

This works but I find it a little unsatisfactory because of two reasons:

1. it requires a full build and a full re-build if anything changes (since bear does not do incremental updates, rebuilding a single file will result in a single file in compile_commands.json.
2. our build system already knows exactly the flags we pass to the compiler so there is no reason to go the roundabout way.

Context

This PR adds to build system a target to create the compile_commands.json file without any unnecessary building. I tried to follow the patterns in the existing makefiles.

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[thufschmitt]

Ooooh that is great!

Tried it locally, and it works like a charm (and generates a compilation database that's semantically equivalent to the one generated by bear).

Left a few minor comments on the form

[thufschmitt]

One of the biggest QOL improvements for contributors (well, for me at least :D).

I've edited the hacking guide to mention it.

Thanks!