config.mk: update to _XOPEN_SOURCE=700L

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>

config.mk

@@ -26,7 +26,7 @@ INCS = -I${X11INC} -I${FREETYPEINC}
 LIBS = -L${X11LIB} -lX11 ${XINERAMALIBS} ${FREETYPELIBS}
 
 # flags
-CPPFLAGS = -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_POSIX_C_SOURCE=200809L -DVERSION=\"${VERSION}\" ${XINERAMAFLAGS}
+CPPFLAGS = -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_XOPEN_SOURCE=700L -DVERSION=\"${VERSION}\" ${XINERAMAFLAGS}
 #CFLAGS   = -g -std=c99 -pedantic -Wall -O0 ${INCS} ${CPPFLAGS}
 CFLAGS   = -std=c99 -pedantic -Wall -Wno-deprecated-declarations -Os ${INCS} ${CPPFLAGS}
 LDFLAGS  = ${LIBS}