diff --git a/config.mk b/config.mk
index 961313d..8d4dbe3 100644
--- a/config.mk
+++ b/config.mk
@@ -10,6 +10,7 @@ LD = $(CC)
 AR = ar
 RANLIB = ranlib
 
+# For NetBSD add -D_NETBSD_SOURCE
 CPPFLAGS = -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_XOPEN_SOURCE=700
 CFLAGS   = -std=c99 -Wall -pedantic
 LDFLAGS  = -s # -lrt