Add Gnulib-provided files for convenience.

These come from Gnulib's Git commit
ae3a0d62f26d8156b403e40d6007475006f3136f, dated 2008-08-19.

build-aux/link-warning.h

@@ -0,0 +1,28 @@
+/* GL_LINK_WARNING("literal string") arranges to emit the literal string as
+   a linker warning on most glibc systems.
+   We use a linker warning rather than a preprocessor warning, because
+   #warning cannot be used inside macros.  */
+#ifndef GL_LINK_WARNING
+  /* This works on platforms with GNU ld and ELF object format.
+     Testing __GLIBC__ is sufficient for asserting that GNU ld is in use.
+     Testing __ELF__ guarantees the ELF object format.
+     Testing __GNUC__ is necessary for the compound expression syntax.  */
+# if defined __GLIBC__ && defined __ELF__ && defined __GNUC__
+#  define GL_LINK_WARNING(message) \
+     GL_LINK_WARNING1 (__FILE__, __LINE__, message)
+#  define GL_LINK_WARNING1(file, line, message) \
+     GL_LINK_WARNING2 (file, line, message)  /* macroexpand file and line */
+#  define GL_LINK_WARNING2(file, line, message) \
+     GL_LINK_WARNING3 (file ":" #line ": warning: " message)
+#  define GL_LINK_WARNING3(message) \
+     ({ static const char warning[sizeof (message)]		\
+          __attribute__ ((__unused__,				\
+                          __section__ (".gnu.warning"),		\
+                          __aligned__ (1)))			\
+          = message "\n";					\
+        (void)0;						\
+     })
+# else
+#  define GL_LINK_WARNING(message) ((void) 0)
+# endif
+#endif