Rake: update task clean to remove bin and build folders

[jbampton]

No description provided.

[dearblue]

The #{MRUBY_ROOT}/build and {MRUBY_ROOT}/bin directories, which are now hardcoded by this PR, can still be replaced.

• Alternatives to the #{MRUBY_ROOT}/build directory can be specified with the MRuby::Build#build_dir= accessor.
• The {MRUBY_ROOT}/bin directory can be replaced by the INSTALL_DIR environment variable.
  NOTE: There is no documentation for the INSTALL_DIR environment variable, so I believe it is a hidden feature.

Also, #{MRUBY_ROOT}/build/repos/#{MRuby::Build#name} contains a git clone of the third party mrbgems.
The git repository is also removed by rake clean with this PR, so people who are in the middle of modifying other people's code may have trouble with rake clean.
NOTE: The MRuby::Build#gem_clone_dir= accessor allows you to specify an alternative, but I believe it is a hidden feature that is not documented.

Therefore, I will create a PR to revert to.

In my opinion, if the content of this PR is needed, it would be better to define it as a different task than rake clean or rake deep_clean.

[matz]

I think it's worth removing the default build and bin directories in the cleaning task. I think it should be in the deep_clean task.

@dearblue why do you think it should be a different task than existing clean tasks?

[dearblue]

This is because the current deep_clean task only removes third-party GEMS directories related to the build target based on the "Build configuration file" at the time.
Unrelated GEMS directories will be kept intact (on a per-build-target basis).

[matz]

The original meaning of deep_clean was “to clean everything generated during build”, not only in the last build, but in all builds. So it's natural to remove bin and build in the task, as default build target directories. I know some files may be generated outside those directories (according to configurations), but it's better than nothing.

Some software uses distclean task for the purpose, but I see no specific reason for separate tasks.

[dearblue]

I understood. If that was the original purpose of deep_clean, I have no disagreement.