As a sometime contributor to Ceph, I can definitely say that no, it is NOT "intrinsically nonportable." The servers which implement the filesystem itself are all userspace processes, which should, in theory, be portable to any POSIX OS. In fact, you can see a few places in the source where someone added #ifdef DARWIN and similar things. There was never a serious effort to get things working well on non-Linux platforms, just due to limited resources and interest. But that may change in the future.
And yes, the kernel component is Linux-only, but there are other ways to access the filesystem.
> (Arguing that Autoconf is awful because it includes shell
> fragments is also vitiated in the presence of a project
> that uses any manually-written shell scripts at all. As
> most do.)
All the standalone shell scripts that are currently in the Ceph server repo are used for testing purposes. That's why they're in directories with names like 'qa', 'test', and so forth. Later on, the project started moving more towards Python for testing... but I digress.
Would you be ok with a build system that required you to write Makefiles in C? Oh, but your project includes .c files, so your arguments are vitiated. Come on, this is absurd, and you know it.