config.mk: improve feature test check

this fixes a crash on NetBSD because it requires -D_XOPEN_SOURCE (strdup, usleep).

thanks k0ga and stateless for reporting and fixing this issue!

config.mk

@@ -23,7 +23,7 @@ INCS = -I${X11INC} -I${FREETYPEINC}
 LIBS = -L${X11LIB} -lX11 ${XINERAMALIBS} ${FREETYPELIBS}
 
 # flags
-CPPFLAGS = -D_BSD_SOURCE -D_POSIX_C_SOURCE=200809L -DVERSION=\"${VERSION}\" ${XINERAMAFLAGS}
+CPPFLAGS = -D_BSD_SOURCE -D_XOPEN_SOURCE=500 -D_POSIX_C_SOURCE=200809L -DVERSION=\"${VERSION}\" ${XINERAMAFLAGS}
 CFLAGS   = -ansi -pedantic -Wall -Os ${INCS} ${CPPFLAGS}
 LDFLAGS  = -s ${LIBS}