HPCC-11336 Support Rcpp 0.11.0 on Ubuntu 14.04

Make finding of libRcpp optional.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #5807 from richardkchapman/R-trusty

HPCC-11336 Support Rcpp 0.11.0 on Ubuntu 14.04

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>