This is a colon-separated list of directories that will get searched for
build definition in order from left to right.

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

This reverts commit 6cf348e4. See
https://github.com/sstephenson/ruby-build/issues/526#issuecomment-37889552.

Closes https://github.com/sstephenson/ruby-build/issues/526.

[ci skip]

Wrap at 80 characters.

These variables let you pass Ruby-specific configure and make options
that will not be passed to dependent packages like libyaml.