SA_NOCLDWAIT is marked as XSI in the posix spec [0] and FreeBSD and NetBSD
seems to more be strict about the feature test macro [1].

so update the macro to use _XOPEN_SOURCE=700L instead, which is equivalent to
_POSIX_C_SOURCE=200809L except that it also unlocks the X/Open System
Interfaces.

[0]: https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/signal.h.html#tag_13_42
[1]: https://lists.suckless.org/dev/2302/35111.html

Tested on:
* NetBSD 9.3 (fixed).
* FreeBSD 13 (fixed).
* Void Linux musl.
* Void Linux glibc.
* OpenBSD 7.2 (stable).
* Slackware 11.

Reported-by: beastie <pufferfish@riseup.net>

Reported by fossy <fossy@dnmx.org>, thanks

The feature test was incorrect:
_POSIX_C_SOURCE=2

"The value 2 or greater additionally exposes definitions for POSIX.2-1992."
http://man7.org/linux/man-pages/man7/feature_test_macros.7.html

A higher value is needed (atleast 1995):
https://pubs.opengroup.org/onlinepubs/9699919799/functions/snprintf.html

FreeBSD feature test macro:
on
https://github.com/freebsd/freebsd/blob/master/include/stdio.h line 297

This was already fixed in dmenu.

This fixes a warning on FreeBSD, reported by Plasmoduck on IRC, thanks.

Building with debug symbols is worthless unless LDFLAGS are manually
adjusted as well.

- optimised drawbar() and arrange() handling for multiple monitors, arrange only arranges the relevant monitors when applicable, same with drawbar
- need to release 5.7.1