Force build-time gem installs to use current JRuby config

The RubyGems we run at build time runs with JRuby 9.4, which causes
it to use properties from that version for things like gem paths
and extension directories. This is related to the extension
disabling change in jruby#8415 and causes the build-time
extension faking to use the wrong path. For example:

  .../gems/shared/extensions/universal-java-25/3.1.0

when it should be

  .../gems/shared/extensions/universal-java/3.4.0

The change here uses RubyGems' cross-compiling capability by
forcing a specific rbconfig.rb to be use, and additionally fakes
out the Ruby API version by setting @ruby_api_version on the Gem
module.

This was discovered while attempting to switch fully to the syslog
gem in jruby#9109, since as a bundled gem it will not be
activated unless we successfully fake-out the extension build.

See also this bug related to the resolv gem installing extensions:

* jruby#8649

Author: headius

.gitignore

@@ -29,7 +29,6 @@ failures
 .debug.properties
 .redcar
 /.rbx/
-build
 build.properties
 build_graph.png
 core/src/main/java/org/jruby/runtime/Constants.java

lib/pom.rb

@@ -213,6 +213,10 @@ def log(message = nil)
 
     log "using jruby #{JRUBY_VERSION}"
 
+    # force platform to match build JRuby
+    Gem.set_target_rbconfig(File.join(File.dirname(__FILE__), "ruby/stdlib/jruby/build/rbconfig.rb"))
+    Gem.instance_variable_set :@ruby_api_version, Gem.target_rbconfig['ruby_version']
+
     target = ctx.project.build.directory.to_pathname
     gem_home = File.join(target, 'rubygems')
     gems = File.join(gem_home, 'gems')
@@ -251,6 +255,7 @@ def build_extensions
         say 'Skipping native extensions.'
 
         FileUtils.mkdir_p File.dirname(@spec.gem_build_complete_path)
+        p @spec.gem_build_complete_path
         FileUtils.touch @spec.gem_build_complete_path
       end
     end

lib/ruby/stdlib/jruby/build/rbconfig.rb

@@ -0,0 +1,13 @@
+# This file is used by JRuby's build to override the RubyGems platform
+
+module RbConfig
+    build_props = {}
+    File.open(File.join(__FILE__, "../../../../../../default.build.properties")) do |file|
+      build_props = java.util.Properties.new
+      build_props.load(file.to_input_stream)
+    end
+
+    CONFIG = {}
+    CONFIG['ruby_version'] = build_props["version.ruby.major"] + '.0'
+    CONFIG['arch'] = 'universal-java'
+end