Use $Config{siteprefix} as default directory for etc.

[carragom]

Fixes #264. This will use $Config{siteprefix} instead of $Config{prefix} to place the etc directory on a default install, to be consistent with the rest of the files. Also it will give priority to the value of install_base over prefix since it seem to be the recommended way in Module::Build's docs.

[theory]

I think it might make sense to check the Module::Build installdirsattribute and select the Config key appropriately. Something like:

my $which = $self->installdirs;
$which = '' if $which eq 'core';
my $prefix = $self->installdirs eq 'core'   ? $Config{prefix}
           : $self->installdirs eq 'site'   ? $Config{siteprefix}   || $Config{prefix}
           : $self->installdirs eq 'vendor' ? $Config{vendorprefix} || $Config{siteprefix} || $Config{prefix};

[carragom]

Makes sense. Here is another stab at it. Code is a little different but should achieve the same. Let me know what you think. By the way, thanks for the great piece of software.