[DRE-maint] Bug#701716: extension_builder: support DH_RUBY_USE_DH_AUTO_INSTALL_DESTDIR

[Pierre Carrier]

Package: gem2deb
Version: 0.3.0

extension_build.rb does not honor DH_RUBY_USE_DH_AUTO_INSTALL_DESTDIR.

Please see https://github.com/pcarrier-forks/gem2deb/commit/cd77436
for a patch (passes tests, used to build packages).
I didn't write any new tests for it.

-- 
Pierre