Add test for ruby-build definitions
Support changing RUBY_BUILD_ROOT

This is to allow specifying an alternate path where `share/ruby-build/*`
definitions reside. Useful in tests as well.
Extra definitions paths via RUBY_BUILD_DEFINITIONS

This is a colon-separated list of directories that will get searched for
build definition in order from left to right.
Merge pull request #613 from sstephenson/definition-paths

Support extra definitions paths via RUBY_BUILD_DEFINITIONS

Closes #609