Add the extopts functionality itself.

Taken from original project git@github.com:githaff/embox.git since
commit 5493551079b39e94ea5ce54d4b0e731c178dbc7a.
Updated gitignore.

Autoreconf-generated configs are now ignored.
Further cleaning autoreconf-generated files.

Build-aux files and INSTALL are removed and putted into ignore since
they are suposed to be overwritten by default files from host system.