Index: head/tools/build/Makefile
===================================================================
--- head/tools/build/Makefile
+++ head/tools/build/Makefile
@@ -25,6 +25,14 @@
 		-I${.CURDIR}/../../lib/libc/include
 .endif
 
+_WITH_REALLOCARRAY!= grep -c reallocarray /usr/include/stdlib.h || true
+.if ${_WITH_REALLOCARRAY} == 0
+.PATH: ${.CURDIR}/../../lib/libc/stdlib
+INCS+=		stdlib.h
+SRCS+=		reallocarray.c
+CFLAGS+=	-I${.CURDIR}/../../lib/libc/include
+.endif
+
 .if empty(SRCS)
 SRCS=		dummy.c
 .endif
Index: head/tools/build/stdlib.h
===================================================================
--- head/tools/build/stdlib.h
+++ head/tools/build/stdlib.h
@@ -0,0 +1,42 @@
+/*-
+ * Copyright (c) 2015 Dimitry Andric <dim@FreeBSD.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ */
+
+#ifndef _LEGACY_STDLIB_H_
+#define	_LEGACY_STDLIB_H_
+
+#include_next <stdlib.h>
+
+__BEGIN_DECLS
+   
+#if __BSD_VISIBLE
+void	*reallocarray(void *, size_t, size_t);
+#endif /* __BSD_VISIBLE */
+
+__END_DECLS
+
+#endif /* !_LEGACY_STDLIB_H_ */