create Build.PL in filemunging phase

this allows other plugins to not just see that this file exists (at early
phases such as file munging), but allow other plugins to add content earlier
(e.g. Devel::CheckLib). The template will be evaluated at the normal time,
once the prereq list is finalized.
fix visible modeline in pod
v0.008

          Build.PL is now created at the correct phase (gather_files), with
          final content determined at the normal time (setup_installer)