package/Makefile.in: introduce TARGET_NLS_{DEPENDENCIES, LIBS}

All packages that can use NLS support will have to depend on
host-gettext, as it provides the tools for generating .mo files from
.po files.

In addition, all packages may need to depend on gettext (and link with
-lintl) if the full-blown gettext implementation is provided by
libintl and not by the C library.

In order to avoid repeating such conditions everywhere, this commit
introduces two variables:

 - TARGET_NLS_DEPENDENCIES, which packages that may use NLS support
   should unconditionally add to their <pkg>_DEPENDENCIES variable.

 - TARGET_NLS_LIBS, which packages can use to pass through LIBS or
   LDFLAGS when the package build system needs help to realize that it
   should link against libintl.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>