Since we switched from autotools to Meson in commit
94592c1406, we don't need to include
`config.h` early to properly enable large file support. Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.
This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.
With a "+" or "-" prefix, the "metadata_to_use" setting can manipulate
the existing default. This way, one can enable `TAG_COMMENT` without
having to list all the other tags.
