Fix building on Cygwin

Achim reported that 2.10.0 no longer builds on Cygwin, while 2.9.4 did.
This was magnificently obscure, at least to this Unix-head.  Initially
it appeared to be an issue with inter-library linking between libman and
libmandb, but 2.9.4 relied on that too, just slightly less so.

The actual problem was that libman only exported
`gl_get_setlocale_null_lock` and not all the other symbols it's supposed
to export, which is because that was explicitly marked for export and no
other symbols were.  Building libman with `-Wl,--export-all-symbols`
fixes this; libman is intended as an aggregation of various helper
functions, including those from Gnulib, and it doesn't make sense to be
especially selective since it's only installed as a private library
anyway.

(I'm still not exactly sure why 2.9.4 didn't do the same thing, but this
seems to be a good enough fix.)

* configure.ac: Set LIBMAN_EXPORT_LDFLAGS to '-Wl,--export-all-symbols'
when linking with GNU ld on Windows platforms.
* lib/Makefile.am (libman_la_LDFLAGS): Add $(LIBMAN_EXPORT_LDFLAGS).
* src/Makefile.am (LIBMAN): Remove $(top_builddir)/gl/lib/libgnu.la; as
a convenience library, this is already incorporated into libman.la, and
specifying it again causes multiple-definition errors on Cygwin.
* src/tests/Makefile.am (get_mtime_LDADD): Likewise.
* NEWS.md: Document this.