Rather than in `config.h`. This guarantees that they’re always defined
at the start of compiling each unit, rather than being defined only
whenever `config.h` is included.

This fixes a build failure where `sys/types.h` was included before
`config.h` was (transitively) included. The former defined
`_DEFAULT_SOURCE`, and then `config.h` unconditionally re-defined it,
causing a compiler warning.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>