Fix Mac OS X build.

src/Makefile.am

@@ -15,9 +15,10 @@ parse.c: parse.peg
 	if which leg >/dev/null 2>&1; then leg -o $@ $^; else touch $@; fi
 
 # _POSIX_C_SOURCE is for SA_RESTART and others
+# _DARWIN_C_SOURCE is for NI_MAXHOST on OS X
 # _XOPEN_SOURCE is for struct sigaction
 # _GNU_SOURCE is for h_errno and gethostbyname-related macros
-___rinetd_CFLAGS = -std=c99 \
-                   -D_XOPEN_SOURCE -D_GNU_SOURCE -D_POSIX_C_SOURCE=200809L \
+___rinetd_CFLAGS = -std=c99 -D_POSIX_C_SOURCE=200809L \
+                   -D_XOPEN_SOURCE -D_GNU_SOURCE -D_DARWIN_C_SOURCE \
                    -Wall -Wextra -Wwrite-strings