Link atomic to ublox_gps

[kenji-miyake]

Currently, if I build this repository with Clang, it causes errors of undefined reference to some __atomic_* functions.

$ CXX=clang++ colcon build
...
/usr/bin/ld: libublox_gps.so: undefined reference to `__atomic_load'
/usr/bin/ld: libublox_gps.so: undefined reference to `__atomic_store'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
gmake[2]: *** [CMakeFiles/ublox_gps_node.dir/build.make:286: ublox_gps_node] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:167: CMakeFiles/ublox_gps_node.dir/all] Error 2

According to the Clang documentation, we have to link it manually.

Clang does not currently automatically link against libatomic when using libgcc_s. You may need to manually add -latomic to support this configuration when using non-native atomic operations (if you see link errors referring to _atomic* functions).

I know that Clang isn't the main target, but as I guess it doesn't affect GCC, I think it's good to link it.

[clalancette]

I don't object to adding the link to atomic here, but I'm guessing doing it this way is going to cause problems. There's a couple of issues that I see:

1. We have no package.xml dependency to bring in atomic, so I'm not sure this is going to be available on all platforms. In particular, the buildfarm uses a heavily restricted environment, and only installs things which are explicitly mentioned in the package.xml.
2. Similarly, we have nothing in the CMakeLists.txt that does a find_package to get this library. So this may not exist.
3. Finally, we heavily discourage the use of these kinds of open variables. The problem is that they cause hard-coded paths in the output, which isn't a problem when you are building from source, but is a problem when we are generating binary packages for distribution.

Overall, I'd say to get this in, we need some kind of package that provides it (listed in the package.xml), some kind of find_package call to find the library, and then a modern CMake target (atomic::atomic) to link against. All of these could be optional one way or another depending on the platform, but this will need looking into.