make: use rtt_gems dir to install gems if it exists.
make: use rtt_gems dir to install gems if it exists.

Signed-off-by: Peter Soetens <4b8373d016f277527198385ba72fda0feb5da015@thesourceworks.com>

orocos-toolchain/utilrb-ext,orocos-toolchain/utilrb-ext