ruby: fix build breakage

Fix build breakage regarding invalid file format of
/usr/lib/libpthread.so
This is because ruby has rpath enabled by default and was throwing in
the runtime paths and pulling in host libraries into the mix.
So disable rpath.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>