Issue #1261 has been updated by Daniel Golle.

File Makefile added
File mkmf.log added

Update:
Including DESTDIR="#{ENV['DESTDIR']}" helps when compiling, and I gained a working shared library from that.
But the Makefile generated by mkmf just prepends the DESTDIR to the prefix which causes a horrible mess when make install is called given another DESTDIR. 
This is a common situation when creating stripped-down binary distributions, as we don't want to include all the headers and development files needed for building a packet.
If you agree that this is supposed to work differently and indeed is a Bug, I'd happily work on mkmf.rb to fix it.
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http://redmine.ruby-lang.org/issues/show/1261

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