[cctbxbb] Boost broken on mac?

[Luc Bourhis]

> If I had a bit of courage, I would try the trunk of clang/llvm, and then perhaps bisect to find where the problem came from.

llvm/clang just cloned from github leads to the same crash in tst_xray.py as current Xcode does (with Boost 1.60 for the record). This makes Richard removal of -ffast-math even more important on OS X. Now it is to be seen how much speed we actually loose…