config.mk: fix POSIX_C_SOURCE macro for feature test for snprintf()

config.mk: fix POSIX_C_SOURCE macro for feature test for snprintf()

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.