Move the default configuration for the MSVC build system to config_default.pl,

and allow using config.pl to override the defaults. config.pl is removed from
the repository, so changes there will no longer show up when doing diff, and
will not prevent switching branches and such things.

config.pl would normally be used to override single values, but if an
old-style config.pl is read, it will override the entire default configuration,
making it backwards compatible.