diff --git a/mk/os-FreeBSD.mk b/mk/os-FreeBSD.mk
index f8431bc3..a4b2bfd3 100644
--- a/mk/os-FreeBSD.mk
+++ b/mk/os-FreeBSD.mk
@@ -12,4 +12,4 @@
 
 include ${MK}/os-BSD.mk
 
-CPPFLAGS+=	-D_POSIX_C_SOURCE=200809L
+CPPFLAGS+=	-D_BSD_SOURCE