Since we switched from autotools to Meson in commit
94592c14, 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.

Don't use the bad `FatalError` library.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

The initgroups() manpage says we need to check for _BSD_SOURCE.  The
thing is that glibc deprecated this macro, and doesn't define it
anymore, effectively breaking all MPD supplementary groups.

The real fix is to check for initgroups() availability at configure
time, instead of relying on the deprecated _BSD_SOURCE macro.

This was broken by 4f29034f.

Haiku does not dump core, it just starts the debugger.