Don't use git to filter files to be included.
Include Java files only when JRUBY_HOME is set.

--- a/concurrent-ruby.gemspec
+++ b/concurrent-ruby.gemspec
@@ -2,8 +2,6 @@
 require File.join(File.dirname(__FILE__ ), 'lib/concurrent-ruby/concurrent/utility/engine')
 
 Gem::Specification.new do |s|
-  git_files = `git ls-files`.split("\n")
-
   s.name             = 'concurrent-ruby'
   s.version          = Concurrent::VERSION
   s.platform         = Gem::Platform::RUBY
@@ -13,12 +11,12 @@
   s.summary          = 'Modern concurrency tools for Ruby. Inspired by Erlang, Clojure, Scala, Haskell, F#, C#, Java, and classic concurrency patterns.'
   s.license          = 'MIT'
   s.date             = Time.now.strftime('%Y-%m-%d')
-  s.files            = [*Dir['lib/concurrent-ruby/**/*.rb'] & git_files,
-                        *Dir['ext/concurrent-ruby/**/*'] & git_files,
+  s.files            = [*Dir['lib/concurrent-ruby/**/*.rb'],
+                        *(Dir['ext/concurrent-ruby/**/*'] if ENV['JRUBY_HOME']),
                         'Rakefile',
                         'Gemfile',
-                        'lib/concurrent-ruby/concurrent/concurrent_ruby.jar'
+                        ('lib/concurrent-ruby/concurrent/concurrent_ruby.jar' if ENV['JRUBY_HOME']),
   ]
   s.extra_rdoc_files = Dir['README*', 'LICENSE*', 'CHANGELOG*']
   s.require_paths    = ['lib/concurrent-ruby']
   s.description      = <<-TXT.gsub(/^ +/, '')
--- a/concurrent-ruby-edge.gemspec
+++ b/concurrent-ruby-edge.gemspec
@@ -2,8 +2,6 @@
 require File.join(File.dirname(__FILE__ ), 'lib/concurrent-ruby-edge/concurrent/edge/version')
 
 Gem::Specification.new do |s|
-  git_files = `git ls-files`.split("\n")
-
   s.name             = 'concurrent-ruby-edge'
   s.version          = Concurrent::EDGE_VERSION
   s.platform         = Gem::Platform::RUBY
@@ -13,7 +11,7 @@
   s.summary          = 'Edge features and additions to the concurrent-ruby gem.'
   s.license          = 'MIT'
   s.date             = Time.now.strftime('%Y-%m-%d')
-  s.files            = Dir['lib/concurrent-ruby-edge/**/*.rb'] & git_files
+  s.files            = Dir['lib/concurrent-ruby-edge/**/*.rb']
   s.extra_rdoc_files = Dir['README*', 'LICENSE*', 'CHANGELOG*']
   s.require_paths    = ['lib/concurrent-ruby-edge']
   s.description      = <<-TXT