Correct ros link issue described in issue #20

This issue is due to the order of link flags. As a result, even if
the flags appear in the command line, some symbols are not loaded.
 (appears e.g. on ubuntu 12.04).
Note that the solution used is the same as the one used in
 pkg-config.cmake, method PKG_CONFIG_USE_LLINK_DEPENDENCY

Related link:
https://github.com/jrl-umi3218/jrl-cmakemodules/issues/20