msmtp: don't try to use IDN feature

autoconf can't detect if AI_IDN is really supported
(http://sourceforge.net/p/msmtp/code/ci/907916e8089ea9303a7c091b763087530c88a063/

),
so a --disable-gai-idn flag was added to explicitly disable the usage
of IDN.

Before this patch, msmtp failed to send mail with the following
output:

  msmtp: cannot locate host smtp.mandrillapp.com: Bad value for ai_flags

[Thomas: tweak commit log.]

Signed-off-by: Anthony Viallard <viallard@syscom-instruments.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>