lib: libatomic do not exist on MacOS

[vooon]

As reported by @eric1221bday, mac do not have libatomic, so fix from #797 breaks compilation.
Workaround: comment-out atomic from target_link_libraries(mavros, ...).

[TSC21]

We should find a way to generalize this workaround. The best (but overkill way) would be to add a .cmake module to search (or set) the TARGET architecture.

[vooon]

I think we may do module that set some variable, what we can later pass to target_link_libraries().
But first may be better to look at existing cmake module library, problem looks common, so it might be solved.

[oceanusxiv]

I don’t know how fine grained you would want the target to be, but cmake does provide flags for OS, compiler, and CPU, check out https://cmake.org/cmake/help/v3.6/manual/cmake-toolchains.7.html

[vooon]

Problem that we also depends on OS version. For example Hydro has similar problem with older GCC.
I also suppose, that newer versions of OSes for ARM may nor require explicit linking with libatomic.
But for now it is needed at least for ARMv7 and ARMv8. AMD64 do not require, and perhaps LLVM don't need explicit libatomic.

[oceanusxiv]

OS version I believe is also part of CMake, there's also a flag for compiler version, https://cmake.org/cmake/help/v3.0/variable/CMAKE_LANG_COMPILER_VERSION.html, however as the page noted it's not guaranteed to defined for every version or compiler, so that may warrant some testing