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

[Glenn Morris]

Package: emacs
Version: 25.0.50

Current master on x86_64 RHEL7:

make maintainer-clean
configure
cd lib
make libgnu.a

Fails with:

  In file included from binary-io.c:3:0:
  binary-io.h: In function 'set_binary_mode':
  binary-io.h:57:10: error: 'O_BINARY' undeclared (first use in this function)

'make libgnu.a' doesn't generate the lib/ version of fcntl.h, which is
what defines O_BINARY.

In contrast, 'make all' (in lib) does.

I noticed this because src/temacs depends on libgnu.a and calls 'make
libgnu.a' to create it (I would like 'make -C src emacs' to work).

Perhaps it should call 'make -C lib all' instead (an Emacs issue);
or perhaps 'make libgnu.a' should generate the files it requires
(a gnulib issue?).