See the log URL.
Did you check the contents of /usr/local/include/fst/float-weight.h ? This error doesn't seem to be a compiler issue.
(In reply to Gleb Popov from comment #1) I see. It looks for a GCC-specific macro __SSE2_MATH__ https://gcc.gnu.org/legacy-ml/gcc-patches/2002-08/msg01345.html that was never added in clang. Thanks for pointing this out!