bug#20894: make libgnu.a doesn't generate everything it requires

[Glenn Morris]

Paul Eggert wrote:

>> (I would like 'make -C src emacs' to work).
>
> *That* we can arrange.
>
>> Perhaps it should call 'make -C lib all' instead (an Emacs issue);
>
> Yes, that's an easy way to arrange things.  I installed that as part
> of commit 319eeeb0fb154a0cd1d36ec33c68029ff9d6c290.  Good enough?

'make -C src emacs' now works, but 'make -j8 -C src emacs' fails in lib with

  GEN      stddef.h
  mv: cannot stat 'alloca.h-t': No such file or directory
  make[2]: *** [alloca.h] Error 1