Consolidation with CDI-PIO (develop)

Generally, this is !11 (merged) rebased onto the develop branch.

This is the first develop-based version accepted by the Buildbot of ICON: https://buildbot.dkrz.de/grid?branch=m300488/cdi-pio-2-integration

config/interface.rb

@@ -2,6 +2,8 @@ require 'mkmf'
 require 'rbconfig'
 if RUBY_VERSION[0,3] == '1.8'
   puts "-I#{Config::expand(CONFIG['archdir'])}"
+elsif RbConfig::CONFIG['MAJOR'].to_i >= 2
+  puts "-I#{RbConfig::CONFIG['rubyhdrdir']} -I#{RbConfig::CONFIG['rubyarchhdrdir']}"
 else
   puts "-I#{RbConfig::CONFIG['rubyhdrdir']} -I#{RbConfig::CONFIG['rubyhdrdir']}/#{RbConfig::CONFIG['arch']}"
 end