obstack: port to gcc -fcheck-pointer-bounds Problem found by 'make check' failure on bleeding-edge coreutils on an MPX-enabled CPU (Intel Core i3-7100U) running GCC (Ubuntu 6.2.0-5ubuntu12), configured via "./configure --enable-gcc-warnings CFLAGS='-mmpx -fcheck-pointer-bounds -g3 -O2' LDFLAGS='-static-libmpx -static-libmpxwrappers'". * lib/obstack.h (__FLEXIBLE_ARRAY_MEMBER): New macro, copied from fts_.h. (struct _obstack_chunk.contents): Use it.