txt-from-texi: add support for variables

txt-from-texi: add support for variables

The support for variables in the text-from-texi script is used to
automatically insert the current version of Window Maker, inherited from
the definition in the AC_INIT command, so it will be always correctly
updated without the need for hand work.

Having this version information is recommended by GNU texinfo because if
the document happens to get distributed outside a package it can help user
avoid problems.

The second use is to define a variable 'emailsupport' which contains the
email address of the development team, so we can also inherit it from the
AC_INIT setting.

The third use is for a special variable 'cctexi2txt' to differentiate
between texi2any and our script, because when using texi2any the title
page is not generated, so we need to duplicate some information in the
source, but we do not want it to appear twice in our generated doc.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>