_GNU_SOURCE is needed for strcasestr().
The macro should be defined before including any headers. This broke
recently because glib.h seems to include string.h now.
If another instance of calls was already running, invoking calls with
`-v` flag would set the verbosity for the newly created process and then
exit if it was not the primary instance.
This helps provider better logs. With this, the user can now run
with `-v` for verbose logs, use more `-v` to be more verbose,
eg `calls -vvvvv` log shall be very verbose
