Add opendkim -O option and StdoutLog config parameter (for use in Docker)

[rafork]

Add the opendkim -O option and the StdoutLog config parameter for use in Docker which expects log messages on standard output.

Fixes issue #153. This version of the pull request is based on the develop branch.

I also fixed another buglet. A log message had been changed to include the algorithm, but it hadn't been added to the corresponding non-NULL test.

I also lowered the minimum required version for autoconf (in a separate commit). It was recently increased from 2.61 to 2.71, but the current Debian stable only has autoconf-2.69. Lowering the minimum required version to 2.69 allows autoreconf to work on Debian. It's probably the case that it would work with an even earlier version.

[henrythasler]

Any chance, the stdout-logging will be made available?