Create an empty Makefile from extconf.rb on unsupported platforms.

@csfrancis & @byroot for review

## Problem

Doing `gem install semian` still fails on unsupported platforms, since it still tries to build the extensions.  Pull #11 tried avoid building the extensions by setting the extensions attribute conditionally in the gemspec, but this gets read at build time.

## Solution

Create a Makefile from extconf.rb that does nothing for the [targets rubygems uses](https://github.com/rubygems/rubygems/blob/3e365282d1fa4210011b3095d685c10483a7a62c/lib/rubygems/ext/builder.rb#L43-L55).  Nothing needs to be written, since rubygems just [moves the contents of the destination directory without looking for any specific files](https://github.com/rubygems/rubygems/blob/3e365282d1fa4210011b3095d685c10483a7a62c/lib/rubygems/ext/ext_conf_builder.rb#L48-L58).