Bug#390712: Work around

[Tim Angus]

This also occurs with exim4-daemon-heavy and my Nokia E70 with EPOC
mail.

Since the problem is in GNUTLS, it can be worked around by using
OpenSSL, which is fortunately simply a case of editing debian/rules
such that OPENSSL := 1.

UNfortunately, this isn't so staightforward with -heavy because
apparently linking it with OpenSSL constitutes a GPL violation. You are
thus forced to build an exim4-daemon-custom (and violate the GPL) in
order to get -heavy's features with OpenSSL.