diff --git a/mk/os.mk b/mk/os.mk
index 2de3bc61..c99f8b6e 100644
--- a/mk/os.mk
+++ b/mk/os.mk
@@ -18,7 +18,7 @@ _LIBNAME!=	${_LIBNAME_SH}
 LIBNAME?=	${_LIBNAME}$(shell ${_LIBNAME_SH})
 RC_LIB=		/$(LIBNAME)/rc
 
-_DEF_SH=	case `uname -s` in Linux) echo "-D_XOPEN_SOURCE=600 -D_BSD_SOURCE";; *) echo;; esac
+_DEF_SH=	case `uname -s` in Linux) echo "-D_BSD_SOURCE -D_XOPEN_SOURCE=600";; *) echo;; esac
 _DEF!=		${_DEF_SH}
 CFLAGS+=	${_DEF}$(shell ${_DEF_SH})