Refactor install.rb

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2428 f4f36d40-9719-0410-9868-dde021a23eda
Add a config option "--ruby-archheader-dir"

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2429 f4f36d40-9719-0410-9868-dde021a23eda
Fix config error with --sdkroot option

an error "mkmf.rb can't find header files for ruby" may
occur at config task.
mkmf.rb tests whether ruby.h and config.h exists. we need
to tell the location of headers under sdk when we build
RubyCocoa with --sdkroot option.

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2430 f4f36d40-9719-0410-9868-dde021a23eda