This issue was solved with changeset r41635.
Tomoyuki, thank you for reporting this issue.
Your contribution to Ruby is greatly appreciated.
May Ruby be with you.
merge revision(s) 41629: [Backport #8533]
* lib/rubygems/ext/builder.rb (Gem::Ext::Builder.make): Pass
DESTDIR via command line to override what's in MAKEFLAGS. This
fixes an installation problem under a package building
environment where DESTDIR is specified in the (parent) command
line. [Fixes GH-327]