Make building of jruby-launcher gem optional

[jayjlawrence]

The Jruby-launcher gem is not compatible with all versions of jruby. As well it may not always be desirable to have this gem automatically installed by ruby-build. Updated jruby-1.6.8 as it requires this per #1292.

It may be preferable to have the jruby-launcher separated out from the build_package_jruby function along the lines of other platforms. This patch tries to follow current code patterns (but I may have misjudged) and to be of minimal impact.

Took the liberty of modifying the code. Was unsure about the test suite.

[nirvdrum]

It looks like you've only applied this to JRuby 1.6.8. Is that the only version that's problematic or is that the only one you've tested with? I'm trying to ascertain whether the other jruby-1.6.x and jruby-1.5.x definitions should also disable the launcher.

[jayjlawrence]

This didn't sit right with me so I had a look at jruby-launcher. We just need a require 'rbconfig' in the extconf to make the underlying issue go away. I've submitted jruby/jruby-launcher#23 to address that. They will need to push a new gem.

Nonetheless I believe that automatically installing jruby-launcher as part of the JRuby build is undesirable. It should be a post build task.

I would recommend phasing it out - announce for 6 months to all jruby installs "Installing jruby-launcher (Phased out after Sept 1, 2019 )" . Then "No longer installing jruby-launcher by default; put it in your Gemfile or other post build tasks"

If you want I can do the above - subject to your adjustments.

[nirvdrum]

I think @headius or @enebo should weigh in on this, since it impacts how many people install and run JRuby.

[mislav]

Thank you, but see #1292 (comment) please.