5 files changed, 12 insertions, 5 deletions
diff --git a/lib/rubygems/ext/builder.rb b/lib/rubygems/ext/builder.rb
index 761505636c..e9244c760c 100644
--- a/lib/rubygems/ext/builder.rb
+++ b/lib/rubygems/ext/builder.rb
@@ -149,6 +149,7 @@ EOF
     extension ||= '' # I wish I knew why this line existed
     extension_dir =
       File.expand_path File.join @gem_dir, File.dirname(extension)
+    lib_dir = File.join @spec.full_gem_path, @spec.raw_require_paths.first
 
     builder = builder_for extension
 
@@ -158,7 +159,7 @@ EOF
       CHDIR_MUTEX.synchronize do
         Dir.chdir extension_dir do
           results = builder.build(extension, @gem_dir, dest_path,
-                                  results, @build_args)
+                                  results, @build_args, lib_dir)
 
           say results.join("\n") if Gem.configuration.really_verbose
         end
diff --git a/lib/rubygems/ext/cmake_builder.rb b/lib/rubygems/ext/cmake_builder.rb
index 17bd6c296e..24531bc75c 100644
--- a/lib/rubygems/ext/cmake_builder.rb
+++ b/lib/rubygems/ext/cmake_builder.rb
@@ -1,7 +1,7 @@
 require 'rubygems/command'
 
 class Gem::Ext::CmakeBuilder < Gem::Ext::Builder
-  def self.build(extension, directory, dest_path, results)
+  def self.build(extension, directory, dest_path, results, args=[], lib_dir=nil)
     unless File.exist?('Makefile') then
       cmd = "cmake . -DCMAKE_INSTALL_PREFIX=#{dest_path}"
       cmd << " #{Gem::Command.build_args.join ' '}" unless Gem::Command.build_args.empty?
diff --git a/lib/rubygems/ext/configure_builder.rb b/lib/rubygems/ext/configure_builder.rb
index 2a542e6bd0..f66e39387a 100644
--- a/lib/rubygems/ext/configure_builder.rb
+++ b/lib/rubygems/ext/configure_builder.rb
@@ -6,7 +6,7 @@
 
 class Gem::Ext::ConfigureBuilder < Gem::Ext::Builder
 
-  def self.build(extension, directory, dest_path, results, args=[])
+  def self.build(extension, directory, dest_path, results, args=[], lib_dir=nil)
     unless File.exist?('Makefile') then
       cmd = "sh ./configure --prefix=#{dest_path}"
       cmd << " #{args.join ' '}" unless args.empty?
diff --git a/lib/rubygems/ext/ext_conf_builder.rb b/lib/rubygems/ext/ext_conf_builder.rb
index 9a656a30a3..6e736d8062 100644
--- a/lib/rubygems/ext/ext_conf_builder.rb
+++ b/lib/rubygems/ext/ext_conf_builder.rb
@@ -10,7 +10,7 @@ require 'tempfile'
 class Gem::Ext::ExtConfBuilder < Gem::Ext::Builder
   FileEntry = FileUtils::Entry_ # :nodoc:
 
-  def self.build(extension, directory, dest_path, results, args=[])
+  def self.build(extension, directory, dest_path, results, args=[], lib_dir=nil)
     tmp_dest = Dir.mktmpdir(".gem.", ".")
 
     t = nil
@@ -44,6 +44,12 @@ class Gem::Ext::ExtConfBuilder < Gem::Ext::Builder
 
         if tmp_dest
           FileEntry.new(tmp_dest).traverse do |ent|
+            # TODO remove in RubyGems 3
+            if lib_dir then
+              libent = ent.class.new lib_dir, ent.rel
+              libent.exist? or ent.copy libent.path
+            end
+
             destent = ent.class.new(dest_path, ent.rel)
             destent.exist? or File.rename(ent.path, destent.path)
           end
diff --git a/lib/rubygems/ext/rake_builder.rb b/lib/rubygems/ext/rake_builder.rb
index 984c54bc1b..2093bcabdd 100644
--- a/lib/rubygems/ext/rake_builder.rb
+++ b/lib/rubygems/ext/rake_builder.rb
@@ -6,7 +6,7 @@
 
 class Gem::Ext::RakeBuilder < Gem::Ext::Builder
 
-  def self.build(extension, directory, dest_path, results, args=[])
+  def self.build(extension, directory, dest_path, results, args=[], lib_dir=nil)
     if File.basename(extension) =~ /mkrf_conf/i then
       cmd = "#{Gem.ruby} #{File.basename extension}"
       cmd << " #{args.join " "}" unless args.empty?