GH398: Add mingw cross-compile, etc.

For all release branches.  It adds travis build support. If you don't
have a config file it uses the default (because we enabled travis for the
project), which uses ruby/rake/rakefiles, and you get confusing "build
still failing" messages.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(cherry picked from commit db9defdf)