Merge pull request #8 from algernon/f/gitignore-update

.gitignore: Add a few more autotools generated files
autogen: add script to run autoreconf.
builtins: Add support for variadic arguments to arith functions

This adds support for variadic arguments to PLUS, MINUS, TIMES, QUOTIENT
and REMAINDER. Tests were also modified to reflect the new behaviour,
and a few new cases were added to handle cases that were not possible
before.

This fixes #3.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>