pkg-autotools: add support for <pkg>_AUTORECONF_ENV

This commit improves the autotools-package infrastructure to support a
<pkg>_AUTORECONF_ENV variable, which allows a package maintainer to
pass additional variables in the environment of the autoreconf
execution. This is useful in the situation where configure.ac contains
shell commands using environment variables that are only executed when
autoreconf is run.

Signed-off-by: Rick Taylor <rick.taylor@cassidian.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>