dev-lang/ruby: fix bootstrap build on uclibc

dev-lang/ruby: fix bootstrap build on uclibc

With a blank LIBPATHENV the ruby build tools fall back to using LD_PRELOAD
to load libruby.so during the build process. With uclibc, preloading is
now optional and not enabled at least in the vanilla stage3 that we
produce. This causes the build to fail since there are no options left
to find libruby.so.

Fixed by adding ${S} to the temporary LD_LIBRARY_PATH during the install
phase. Thanks to blueness for the original patch.

Fixes bug 564272

Package-Manager: portage-2.2.28