I guess this regression was introduced in r37016 by dropping this line [1] in mkmf. Not sure what was the intention, since creating directory seems unrelated to the timestamp dependencies IMO
This issue was solved with changeset r39366.
Kenta, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.
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.
As Vit Ondruch has noted, it seems to be a bug in the Makefile generated by mkmf.rb as ruby 1.9.3 without Murata-san's patch correctly installs lib/msgpack.bundle.
Nobu, can you check it?
Just to be safe, I committed Murata-san's patch and will open a backport request.