Add Compilation Database support

[sim642]

Closes #406.

So far I've just tested it with this tiny example CMake project I set up: https://github.com/sim642/goblint-action-test/tree/compilation-database/cmake-project. Nevertheless, it has a library component and also has custom -D preprocessing arguments for the build.

Changes

1. Parsing of compilation databases.
2. Preprocessing of compilation database command objects, either from command (from CMake) or arguments (from Bear, compiledb).
3. Output (compilation database) preprocessed files into new .goblint/preprocessed/ subdirectory. Over time we should consolidate all of Goblint's generated data (e.g. incremental_data) there for ease of management instead of scattering our files in top-level directories.
4. Support for directories to be analyzed, e.g. goblint . for current directory. Looks up compilation database or Makefile in the directory and uses that; doesn't recurse into individual C files because that's extremely unlikely to be correct on any real project.
5. Change cppflags option from string to string list. This integrates better with command objects with arguments (otherwise we'd have to parse shell commands a la Python's shlex).
6. autogen support by @stilscher. It can be there although I believe Goblint shouldn't be in charge of configuring the project that it's analyzing (and sometimes you might want to use an alternative configuration instead of the default we currently call). Instead the user should have configured the project as desired (including with CMake) and we analyze just that (e.g. the compilation database or Makefile).

TODO

• Try on real projects to see if it works.

[sim642]

I also added support for analyzing directories, e.g. goblint .. This will be useful for GobPie, so it can just call Goblint on the enabled directory instead of duplicating the logic to figure out what (compilation database or Makefile) to use.