Preserve environment when shelling out to gem build

lib/bundler/gem_helper.rb

@@ -76,7 +76,7 @@ def install
     def build_gem
       file_name = nil
       gem = ENV["BUNDLE_GEM"] ? ENV["BUNDLE_GEM"] : "gem"
-      sh(%W[#{gem} build -V #{spec_path}]) do
+      sh(%W[#{gem} build -V #{spec_path}], "RUBYOPT" => "-I#{File.expand_path("..", __dir__)}") do
         file_name = File.basename(built_gem_path)
         SharedHelpers.filesystem_access(File.join(base, "pkg")) {|p| FileUtils.mkdir_p(p) }
         FileUtils.mv(built_gem_path, "pkg")
@@ -180,19 +180,19 @@ def name
       gemspec.name
     end
 
-    def sh(cmd, &block)
-      out, status = sh_with_status(cmd, &block)
+    def sh(cmd, env = {}, &block)
+      out, status = sh_with_status(cmd, env, &block)
       unless status.success?
         cmd = cmd.shelljoin if cmd.respond_to?(:shelljoin)
         raise(out.empty? ? "Running `#{cmd}` failed. Run this command directly for more detailed output." : out)
       end
       out
     end
 
-    def sh_with_status(cmd, &block)
+    def sh_with_status(cmd, env = {}, &block)
       Bundler.ui.debug(cmd)
       SharedHelpers.chdir(base) do
-        outbuf = IO.popen(cmd, :err => [:child, :out], &:read)
+        outbuf = IO.popen(env, cmd, :err => [:child, :out], &:read)
         status = $?
         block.call(outbuf) if status.success? && block
         [outbuf, status]