add installation information for the tarball users

[Bruno Haible]

Now that you start to make tarballs, you need to distinguish two audiences:
  (1) the users who get the tarball and merely want to build and install it,
  (2) the developers who want to get their hands dirty.

The audience of the HACKING file is (2).

(1) fetch the tarball only. It does NOT contain the HACKING file. But some of
the info from the HACKING file is necessary for this wider audience, namely
the installation information, with a reference to the list of dependencies
(prerequisite packages).

Here is a proposed patch. It makes sure that the INSTALL file contains the
installation information, with a separate DEPENDENCIES file for the
prerequisite packages (like in gnulib, gettext, libunistring, etc.).
I also listed the package names in some distros, to make it easier for
the user. (For example, the distro package name for the Boehm GC is not
easy to find out.)

0001-Add-installation-information-for-the-tarball-users.patch
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