redefinition of 'mallocx'

Hi,
Im working on project using FBVector and FBString and while trying to compile with jemalloc i get errors like: ```redefinition of 'mallocx' as different kind of symbol```, while defining ```USE_JEMMALOC```.
without defining ```USE_JEMMALOC``` the function ```folly::usingJEMalloc()``` return false.

Im using mac osx sierra
Apple LLVM version 8.1.0
jemalloc 5.0.1
folly - latest from brew

I tried solutions from this issue without success #635