Update Gnulib to 6835fc458f30b94f15d69c35a79cbc2dfabe2d06.

lib/assure.h

@@ -0,0 +1,37 @@
+/* Run-time assert-like macros.
+
+   Copyright (C) 2014-2016 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify
+   it under the terms of the GNU Lesser General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Written by Paul Eggert.  */
+
+#ifndef _GL_ASSURE_H
+#define _GL_ASSURE_H
+
+#include <assert.h>
+
+/* Check E's value at runtime, and report an error and abort if not.
+   However, do nothng if NDEBUG is defined.
+
+   Unlike standard 'assert', this macro always compiles E even when NDEBUG
+   is defined, so as to catch typos and avoid some GCC warnings.  */
+
+#ifdef NDEBUG
+# define assure(E) ((void) (0 && (E)))
+#else
+# define assure(E) assert (E)
+#endif
+
+#endif