--- a/tool/rbinstall.rb.~1~	2019-01-24 02:00:07.000000000 +0100
+++ b/tool/rbinstall.rb	2019-02-03 08:39:49.778007118 +0100
@@ -806,7 +806,7 @@
 end
 
 def install_default_gem(dir, srcdir)
-  gem_dir = Gem.default_dir
+  gem_dir = ENV['GEM_DESTDIR']
   directories = Gem.ensure_gem_subdirectories(gem_dir, :mode => $dir_mode)
   prepare "default gems from #{dir}", gem_dir, directories
 
@@ -844,7 +844,7 @@
 end
 
 install?(:ext, :comm, :gem, :'bundled-gems') do
-  gem_dir = Gem.default_dir
+  gem_dir = ENV['GEM_DESTDIR']
   directories = Gem.ensure_gem_subdirectories(gem_dir, :mode => $dir_mode)
   prepare "bundled gems", gem_dir, directories
   install_dir = with_destdir(gem_dir)