[CMake] set RPATH for plugins

to fix debian packages creation on ROS buildfarm:

00:12:11 dpkg-shlibdeps: error: cannot find library libtracer.so.4.3.1 needed by
debian/ros-noetic-dynamic-graph/opt/ros/noetic/lib/dynamic-graph-plugins/libtracer-real-time.so.4.3.1
(ELF format: 'elf64-x86-64' abi: '0201003e00000000'; RPATH: '')

[CMake] DYNAMIC_GRAPH_PLUGINDIR is now relative

Tracer: Adding lock to make it threadsafe

For now, the plugins can only be loaded from python, so we just need the
wrap.so to be able to find its plugin.so. For this, there is no need for
a unique hard-coded absolute path to provide an RPATH to
libdynamic-graph.so.

And actually, that RPATH was not even set.

ref. https://github.com/boostorg/test/commit/235acea2fa4b1eb7580acbfcce94d696c86bb207#diff-9131a06d2a1a6d0c877c9b7909acbbde

Using both ADD_PROJECT_DEPEDENCY & FIND_PACKAGE for Boost breaks uses of
${Boost_LIBRARIES}: only the last call to one ore the other is taken
into account.

I suggest to use ${Boost_LIBRARIES} for exported ones, and eg.
${Boost_UNIT_TEST_FRAMEWORK_LIBRARY} for the private ones.

Release of version 4.2.2.

[Tests] fix compilation warnings

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Resolve the unused-but-set-variable warning from res value to have a stable release in ros buildfarm

Uncomment this line to resolve the unused-but-set-variable warning to have a stable release in ros buildfarm

Add operations to the unused variables to silence the unused variable warnings in order to have a stable release of dynamic-graph into the ros buildfarm