g_setenv() and g_unsetenv() can never be thread-safe, so emit a message if
they are used after any threads have been spawned.

This can’t catch interactions between setenv() and g_thread_new(), or
between g_setenv() and pthread_create(), but it’ll catch most
misbehaviour in GLib-centric code.

Currently, the message is a `g_debug()` call. Eventually, I’d like to
upgrade it to a `g_warning()`, but there are a number of GLib tests
which call g_setenv() after threads have been created, and they need to
be fixed first. Emitting a `g_debug()` message gives people an
opportunity to start fixing their code.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Helps: #715