[fetchmail][PATCH] support forced selection of fallback MDA

I needed to force FALLBACK_MDA to be /usr/sbin/sendmail for Debian, so I
took the time to make it a proper configure.in target.

Patch attached, apply against 5.7.4.

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh

conf.patch
Description: Text document