HPCC-9016 Centos 5 build error finding R libraries

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4154 from richardkchapman/find-R

HPCC-9016 Centos 5 build error finding R libraries

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>