infra/pkg-perl: add possibility to pass extra env at configure time

Some perl packages may use environment variables as a hint to know how
to be configured.

That's for example the case for perl-net-ssleay that uses
OPENSSL_PREFIX, if it is set in the environment, as the prefix to
openssl.

Add a new variable that packages can set if they need extra environment
variables. Update the manual accordingly.

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr: enhance the commit log]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>