prelude.c is an automatically generated file by template/prelude.c.tmpl.
However it does not contain any required functions. So remove it from
dependency.

Also miniprelude.c is included by mini_builtin.c and does not need
to make miniprelude.o.

`gem_prelude.rb` is not compiled yet. This patch compile it to
compiled binary.

The script in prelude.rb was embed in MRI to load it (eval this
script at everyboot).
This commit change the loading process of prelude.rb. MRI doesn't
eval a script, but load from compiled binary with builtin feature.

So that Init_prelude() does not load `prelude.rb` now.

FreeBSD make works differently with `-j` option.

> -j	max_jobs
> 	Specify the maximum number of jobs that `make` may have running
>	at any one time.  The value is saved in `.MAKE.JOBS.` Turns
>	compatibility mode off, unless the `B` flag is also specified.
>	When compatibility mode is off, all commands associated with a
>	target are executed in a single shell invocation as opposed to
>	the traditional one shell invocation per line.  This can break
>	traditional scripts which change directories on each command
>	invocation and then expect to start with a fresh environment on
>	the next line.  It is more efficient to correct the scripts
>	rather than turn backwards compatibility on.

Stop using exit, cd, exec in middle of commands.

Instead of reading from the files by the full-path at runtime.  As
rbinc files need to be included in distributed tarballs, the
full-paths at the packaging are unavailable at compilation times.