As of btrfs-progs 3.18, btrfs has the possibility to disable backtrace support. This is required to function with musl-libc. I personally think the backtrace support should be off by default, and only enabled through a USE flag. 3.19 does not compile on musl-libc based systems, because configure complains about having no backtrace support. Setting EXTRA_ECONF="--disable-backtrace" resolves the issue.
+ 19 Apr 2015; Mike Gilbert <floppym@gentoo.org> btrfs-progs-3.19.1.ebuild, + btrfs-progs-9999.ebuild: + Disable backtrace on non-glibc systems.