SConstruct: log configure commands to config.log
SConstruct: log configure commands to config.log

Forgot that with `TryAction`, configure commands and their outputs were
being logged in `config.log`, and the previous commit failed to retain
this ability.

New function `config_command` mimics the logging, with the modification
that output lines are prefixed with `->` so they stand out a bit more.

lightmare/mapnik,tomhughes/mapnik,mapnik/mapnik,lightmare/mapnik,naturalatlas/mapnik,naturalatlas/mapnik,tomhughes/mapnik,mapnik/mapnik,lightmare/mapnik,naturalatlas/mapnik,mapnik/mapnik,lightmare/mapnik,mapnik/mapnik,naturalatlas/mapnik,tomhughes/mapnik,tomhughes/mapnik