Compatibility with conda builds

Hi @tfoote, Sorry - I opened this PR in a rush and should have given more explanation. The find_package(Threads) and linking to {CMAKE_THREAD_LIBS_INIT} pulls in pthread. The link to rt is also necessary. On my Ubuntu installation of ROS melodic, buffer_server links indeed to both pthread and rt, although it is currently not explicitly linked. I can only guess that it's being pulled in from another dependency somehow, but it is better to link explicitly:

ldd /opt/ros/melodic/lib/tf2_ros/buffer_server
    linux-vdso.so.1 (0x00007ffeef2ff000)
    libtf2_ros.so => /opt/ros/melodic/lib/libtf2_ros.so (0x00007fe9fa705000)
    libactionlib.so => /opt/ros/melodic/lib/libactionlib.so (0x00007fe9fa4e4000)
    libroscpp.so => /opt/ros/melodic/lib/libroscpp.so (0x00007fe9fa14f000)
    librosconsole.so => /opt/ros/melodic/lib/librosconsole.so (0x00007fe9f9f19000)
    libtf2.so => /opt/ros/melodic/lib/libtf2.so (0x00007fe9f9ce5000)
    libroscpp_serialization.so => /opt/ros/melodic/lib/libroscpp_serialization.so (0x00007fe9f9ae2000)
    librostime.so => /opt/ros/melodic/lib/librostime.so (0x00007fe9f98c2000)
    libboost_system.so.1.65.1 => /usr/lib/x86_64-linux-gnu/libboost_system.so.1.65.1 (0x00007fe9f96bd000)
    libboost_thread.so.1.65.1 => /usr/lib/x86_64-linux-gnu/libboost_thread.so.1.65.1 (0x00007fe9f9498000)
    libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007fe9f9279000)
    libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007fe9f8ef0000)
    libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007fe9f8cd8000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007fe9f88e7000)
    libxmlrpcpp.so => /opt/ros/melodic/lib/libxmlrpcpp.so (0x00007fe9f86c8000)
    libcpp_common.so => /opt/ros/melodic/lib/libcpp_common.so (0x00007fe9f84bd000)
    libboost_chrono.so.1.65.1 => /usr/lib/x86_64-linux-gnu/libboost_chrono.so.1.65.1 (0x00007fe9f82b8000)
    libboost_filesystem.so.1.65.1 => /usr/lib/x86_64-linux-gnu/libboost_filesystem.so.1.65.1 (0x00007fe9f809e000)
    libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007fe9f7d00000)
    librosconsole_log4cxx.so => /opt/ros/melodic/lib/librosconsole_log4cxx.so (0x00007fe9f7ae5000)
    librosconsole_backend_interface.so => /opt/ros/melodic/lib/librosconsole_backend_interface.so (0x00007fe9f78e3000)
    liblog4cxx.so.10 => /usr/lib/x86_64-linux-gnu/liblog4cxx.so.10 (0x00007fe9f751a000)
    libboost_regex.so.1.65.1 => /usr/lib/x86_64-linux-gnu/libboost_regex.so.1.65.1 (0x00007fe9f7212000)
    libconsole_bridge.so.0.4 => /usr/lib/x86_64-linux-gnu/libconsole_bridge.so.0.4 (0x00007fe9f700d000)
    librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007fe9f6e05000)
    /lib64/ld-linux-x86-64.so.2 (0x00007fe9fabec000)
    libapr-1.so.0 => /usr/lib/x86_64-linux-gnu/libapr-1.so.0 (0x00007fe9f6bd0000)
    libaprutil-1.so.0 => /usr/lib/x86_64-linux-gnu/libaprutil-1.so.0 (0x00007fe9f69a5000)
    libicui18n.so.60 => /usr/lib/x86_64-linux-gnu/libicui18n.so.60 (0x00007fe9f6504000)
    libicuuc.so.60 => /usr/lib/x86_64-linux-gnu/libicuuc.so.60 (0x00007fe9f614c000)
    libuuid.so.1 => /lib/x86_64-linux-gnu/libuuid.so.1 (0x00007fe9f5f45000)
    libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007fe9f5d41000)
    libcrypt.so.1 => /lib/x86_64-linux-gnu/libcrypt.so.1 (0x00007fe9f5b09000)
    libexpat.so.1 => /lib/x86_64-linux-gnu/libexpat.so.1 (0x00007fe9f58d7000)
    libicudata.so.60 => /usr/lib/x86_64-linux-gnu/libicudata.so.60 (0x00007fe9f3d2e000)

Therefore, the PR can be summarized by explicitly linking to the rt and pthread dependencies, rather than relying on other dependencies to pull in rt and pthread.

/cc @wolfv @traversaro who might give better explanations ;)

ros / geometry2

Compatibility with conda builds #500