I'm not observing the problem on Fedora 23 with gcc (GCC) 5.3.1 20151207 (Red
Hat 5.3.1-2).
Can you reproduce the problem this way?
./gnulib-tool --dir foo --create-testdir unistd
cd foo
./configure
make
make check
As I understand it, GCC 5.3.1 should not need a substitute stddef.h. So if your
system is creating one, you need to figure out why ./configure decided that
stddef.h wasn't working. Look in config.log; it should say something like this:
configure:7608: checking for max_align_t
[ a lot of chatter ]
configure:7608: result: yes
