diff --git a/tools/build/Makefile b/tools/build/Makefile
index c7875622b042..71a38534f7ac 100644
--- a/tools/build/Makefile
+++ b/tools/build/Makefile
@@ -78,7 +78,7 @@ SYSINCS+=	stat.h
 SRCS+=		futimens.c utimensat.c
 .endif
 
-_WITH_EXPLICIT_BZERO!= grep -c explicit_bzero ${HOST_INCLUDE_ROOT}/strings.h || true
+_WITH_EXPLICIT_BZERO!= cat ${HOST_INCLUDE_ROOT}/strings.h ${HOST_INCLUDE_ROOT}/string.h | grep -c explicit_bzero || true
 .if ${_WITH_EXPLICIT_BZERO} == 0
 # .PATH: ${SRCTOP}/sys/libkern
 # Adding sys/libkern to .PATH breaks building the cross-build compat library