fix build instructions for "On *nix, from the tarball"

[starius]

Github now has Downloads and Releases as two separate entities. There are no links to Downloads from the main page of the repo. "Releaes" currently are simple snapshots of tags. In particular, files in releases are not autoreconf'ed, so they lack file "configure". Downloads are outdated.

Look in the instructions:

On *nix, from the tarball:

    $ ./configure && make && sudo make install

Obviously these instructions do not work for tarball from Releases.

How to solve:

• change INSTALL, add autoreconf step, or
• make complete release on Github. It allows to upload tarball (with autoreconf'ed files)