Merge the re-ordering of when in the build write_buildcustomize.pl is run.

write_buildcustomize.pl is now run as part of the rule that builds miniperl.
Hence if you've built miniperl, you've now always got lib/buildcustomize.pl
too, which simplifies Makefile dependency rules.

write_buildcustomize.pl is now more robust against errors, and miniperl will
abort if a corrupt lib/buildcustomize.pl is found.

None of these changes should have any visible effect outside of the build.