[ng] cleanup: drop support for DJGPP, Windows 95/98/ME, MS-DOS

[ng] cleanup: drop support for DJGPP, Windows 95/98/ME, MS-DOS

See also:
<http://lists.gnu.org/archive/html/automake-ng/2012-05/msg00104.html>

* automake.in (BEGIN): Don't override $ENV{SHELL} for DJGPP.
* bootstrap: Don't special case the default value for BOOTSTRAP_SHELL
on DJGPP.  While we are at, allow that variable to be overridden from
the environment.
* configure.ac (MODIFICATION_DELAY): Define to '2' unconditionally, as
we don't have to cater to quirks of Windows 95/98/ME anymore.
* lib/am/texibuild.am: Remove support for '*.iNN' files used on DJGPP.
* lib/am/texinfos.am: Likewise.
* NG-NEWS: Update.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>