allow RUBYLIB in build scripts

These checks were added to guard against the case where a user might
have e.g. "lib:test" in their RUBYLIB at build time, which would result
in erraneously loading Rubinius' local `lib/rbconfig.rb` or
`lib/rubygems.rb` instead of ones from MRI that's doing the build.

Since `lib/rbconfig.rb` already has a guard against this, and MRI 1.9+
loads `rubygems.rb` automatically at runtime, tripping over the guard in
`rbconfig.rb` immediately before the RUBYLIB check even happens, the
explicit `ENV["RUBYLIB"]` check is both unnecessary and unwanted since
it prevents valid uses of RUBYLIB.

Reverts af5cae6519169841ca8bc97d7501e2ac707b47d7,
        16d3c35a46455157581188f6766b722c98c659fa

References d4793b518bfd8f6c52553ff5694263c577230359
Moved lib to library.
Merge remote-tracking branch 'mislav/allow-rubylib'