[ruby/reline] Check WITH_VTERM env to add vterm gem as dependency

ruby/reline@27b689a7e2

Author: aycabta

lib/reline/reline.gemspec

@@ -24,6 +24,6 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'rake'
   spec.add_development_dependency 'test-unit'
   is_unix = RUBY_PLATFORM =~ /(aix|darwin|linux|(net|free|open)bsd|cygwin|solaris|irix|hpux)/i
-  spec.add_development_dependency 'vterm', '>= 0.0.5' if is_unix
+  spec.add_development_dependency 'vterm', '>= 0.0.5' if is_unix && ENV['WITH_VTERM']
   spec.add_development_dependency 'yamatanooroti', '>= 0.0.6'
 end