I reported this problem upstream at:
but it looks like upstream is lame. No new commits since Sep 13, 2013.
In a nutshell, lib/fdbuf/fdobuf.h references uid_t and gid_t which are defined in unistd.h by POSIX. Because these definitions are indirectly pulled in in glibc, it works there, but on other c standard libs, like musl, it fails. The fix is trivial, just add `#include <unistd.h>` to fdobuf.h.
This change fixes the musl issue and is harmless on glibc and uclibc.
Can I add this without a revbump?
My brain is going soft!
*** This bug has been marked as a duplicate of bug 538482 ***