diff --git a/share/mk/bsd.sys.mk b/share/mk/bsd.sys.mk
--- a/share/mk/bsd.sys.mk
+++ b/share/mk/bsd.sys.mk
@@ -321,9 +321,12 @@
 .endif
 CFLAGS+=	${SSP_CFLAGS}
 .endif # SSP
-.if ${FORTIFY_SOURCE} > 0
-CFLAGS+=	-D_FORTIFY_SOURCE=${FORTIFY_SOURCE}
-CXXFLAGS+=	-D_FORTIFY_SOURCE=${FORTIFY_SOURCE}
+
+.if empty(CFLAGS:M-D_FORTIFY_SOURCE*)
+CFLAGS+=	-D_FORTIFY_SOURCE=${FORTIFY_SOURCE.${.IMPSRC:T}:U${FORTIFY_SOURCE}}
+.endif
+.if empty(CXXFLAGS:M-D_FORTIFY_SOURCE*)
+CXXFLAGS+=	-D_FORTIFY_SOURCE=${FORTIFY_SOURCE.${.IMPSRC:T}:U${FORTIFY_SOURCE}}
 .endif
 
 # Additional flags passed in CFLAGS and CXXFLAGS when MK_DEBUG_FILES is