They're only released once. No release is changed after its release. Once it's out, it's out. The archive is never touched again.
When a release is made, the source code is exported, archived and uploaded to the library web-site. To accidentally change that code, I would have to accidentally repeat that manual process. It's the same with github - I actually use SVN at home, and I make a new repos per release and upload the code, once, to that repos. Github is a release only, write-once source control system, for me.
> I'm going to have to test either way when I integrate a new version of your code
Well, now I'm confused. If that's so, what's the problem? you keep using the old code, it's not changed, either the API or the library binary, so you don't have to test that; and for any new code, you can use the new release version. To the extent the APIs are unchanged, moving fully from one release to another is a matter of issuing a global search-and-replace for the string "liblfdsNNN" to "liblfdsMMM".
> Given this supports only 2 processors and ConcurrencyKit > supports 5
The library supports on Linux almost every processor GCC offers atomic instrincs for - ARM32, IA64, POWERPC32/64, SPARC32/64, MIPS32/64, x64 and x86. I'm pretty sure the library does not support Alpha, however. Of those platforms, I only have access to actual hardware for ARM32, MIPS32 and x64, so only those platforms actually have the test code compiled and run and being seen to pass.
On Windows, support is provided for ARM32, x86, x64 and IA64. Again, of these, I only have access to x64.