Check JRuby version without accessing Gem

[headius]

When running with --disable-gems, JRuby still needs to be able to load FFI from stdlib. Accessing the Gem namespace here requires that gems not be disabled.

This patch uses a simple split and integer comparison to compare the first two digits of the JRuby version with 9.3.

See #763 for the original code.

This patch is necessary for JRuby to fully source the FFI Ruby files from the gem (jruby/jruby#6150). Example failures here are due to the MRI suites running without RubyGems enabled, but they end up needing FFI for internal JRuby functionality.

https://travis-ci.org/github/jruby/jruby/builds/729197020

So close!

[headius]

The TruffleRuby code might want to do the same.

[headius]

The two specific failure cases I see in JRuby CI:

• Our rbconfig/sizeof.rb requires FFI to enumerate native type widths.

org/jruby/RubyModule.java:3830:in `const_missing': uninitialized constant Gem (NameError)
	from /home/travis/build/jruby/jruby/lib/ruby/stdlib/ffi.rb:11:in `<main>'
	from org/jruby/RubyKernel.java:1043:in `require'
	from /home/travis/build/jruby/jruby/lib/ruby/stdlib/rbconfig/sizeof.rb:2:in `<main>'
	from org/jruby/RubyKernel.java:1043:in `require'
	from /home/travis/build/jruby/jruby/test/mri/lib/envutil.rb:11:in `<main>'
	from org/jruby/RubyKernel.java:1043:in `require'
	from org/jruby/RubyKernel.java:1071:in `require_relative'
	from /home/travis/build/jruby/jruby/test/mri/lib/test/unit.rb:8:in `<main>'
	from org/jruby/RubyKernel.java:1043:in `require'
	from test/mri/runner.rb:13:in `<main>'

• FFI is loaded at boot and used on Windows to implement symlink support and on Solaris to implement flock.

NameError: uninitialized constant Gem
  const_missing at org/jruby/RubyModule.java:3830
         <main> at D:/a/jruby/jruby/lib/ruby/stdlib/ffi.rb:11
        require at org/jruby/RubyKernel.java:1043
         <main> at uri:classloader:/jruby/kernel/file.rb:6
           load at org/jruby/RubyKernel.java:1078
         <main> at file:///D:/a/jruby/jruby/lib/jruby.jar!/jruby/kernel.rb:27

The latter usage could possibly delay booting FFI, but either way once it's needed it needs to be loadable without RubyGems present.

[eregon]

In TruffleRuby I did not hit this issue because require "ffi" loads https://github.com/oracle/truffleruby/blob/4b5eda1a04e669d358bdd2eca6de5abba8fbb8cb/lib/truffle/ffi.rb if there was no gem "ffi", version before (or if RubyGems is disabled).
It has the drawback to not load the latest ffi gem with only require "ffi" (needs gem "ffi" or bundle exec).

Making FFI a default gem sounds nice.

[headius]

Patch has been updated to use a more terse version and remove remaining references to Gem.

[eregon]

Looks good to me.
@larskanis Could you merge? (maybe @headius needs a release too?)

[headius]

Once there's a release with these changes I can flip the switch to make ffi a default gem in JRuby and delete our copies of the ruby files.