Merge trunk revision: 39366

* lib/rubygems/installer.rb (build_extensions):  Create extension
  install destination before building extension.  Patch by Kenta Murata.
  [ruby-trunk - Bug #7897]
* test/rubygems/test_gem_installer.rb:  Test for the above.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@39392 b2dd03c8-39d4-4d8f-98ff-823fe69b080e