Add installation information for the tarball users and from a git checkout.

Add installation information for the tarball users and from a git checkout.

We want to have an INSTALL file that contains installation instructions for
the tarball users. These instructions are necessarily package specific,
because they need to list the prerequisites or reference the DEPENDENCIES
file.

Additionally, many people who are not aware that tarballs are the primary
means for source code distribution of GNU packages check out the git repository
and then expect to see instructions for installing from that git checkout
in the INSTALL file. See e.g. <https://savannah.gnu.org/bugs/?62196>.

Unfortunately, both Automake (with strictness option = 'gnu') and 'bootstrap'
both contain code to copy the generic (not package specific) installation
instructions to the file 'INSTALL'. This is not what we want, so we disable
it. (Other packages handle this issue by putting part of the installation
instructions into the README file. This is suboptimal, because this is not
what the README file is for.)

* autogen.sh: Import INSTALL.generic from gnulib.
* INSTALL: Entirely rewritten, based on poke/INSTALL.
* Makefile.am (EXTRA_DIST): Add INSTALL.generic.