Generate lib/buildcustomize.pl at the same time as the miniperl executable.

Doing them together ensures that we always have lib/buildcustomize.pl
available. This simplifies things. The seemingly cranky ordering of having
miniperl notionally depend on lib/buildcustomize.pl, and the rule for
lib/buildcustomize.pl actually also building miniperl permits the rest of
the Makefile to depend on (the obvious) miniperl, not the obscure
lib/buildcustomize.pl
Remove the Icwd Makefile macros as lib/buildcustomize.pl now sets this up.

Now that lib/buildcustomize.pl is built at the same time as building
miniperl, it will always have added paths to Cwd into @INC. Hence there's no
longer a need for Makefile macros to do this.

On Win32 we can't eliminate $(ICWD) completely as it's also being used for some
invocations of the (real) perl binary. Only miniperl loads buildcustomize.pl
to set up @INC to include paths for the initial locations of modules such
as Cwd.
write_buildcustomize.pl no longer writes to STDOUT

write_buildcustomize.pl now opens lib/buildcustomize.pl itself, instead of
writing to STDOUT and relying on the Makefile to set up redirection. This
means that an empty lib/buildcustomize.pl is not created if
write_buildcustomize.pl fails to compile (for whatever reason), and permits
write_buildcustomize.pl to delete (or attempt to delete) the output file if
it detects an error.

Hard code the output file name (lib/buildcustomize.pl), as it's the same on
all platforms, and @ARGV is already used to optionally pass a directory for
write_buildcustomize.pl to change to before running.

Experimentation suggests that various make utilities don't delete a file
created by redirection even if an error occurs. Hence this should be more
robust.

Add -f to the miniperl commandline when running write_buildcustomize.pl to
avoid reading in any existing lib/buildcustomize.pl. write_buildcustomize.pl
doesn't need the setup provided by lib/buildcustomize.pl, and running it
might cause errors which prevents writing out a correct version, making an
incomplete build harder to recover from.
write_buildcustomize.pl now test loads the generated lib/buildcustomize.pl

And deletes it if it encounters an error whilst loading it. A non-functional
lib/buildcustomize.pl will cause the build to fail with seemingly unrelated
errors. Deleting it and exiting with an error should make the cause of build
failures obvious.
In miniperl, if lib/buildcustomize.pl fails to load, exit with an error.

Unlike sitecustomize.pl, if lib/buildcustomize.pl exists, it should not fail.
If it does, it should be reported immediately as a build failure.

This makes the build process more robust.
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.