ruby: extern linkage portability improvement

Some C compilers are stricter when it comes to (tentative) definition
of a variable -- in those compilers introducing variable without 'extern'
keyword always allocates new 'storage' to the variable and linking all
these modules fails due to duplicate symbols.

This is reimplementation of Charlie Allom's patch:
id:"1336481467-66356-1-git-send-email-charlie@mediasp.com",
written originally by Ali Polatel. This version has
more accurate commit message.