update the README about the compiler bug
update the README about the compiler bug

After more testing, it appears that the bug is not specific to Apple (I
reproduced it on Linux as well): it is a bug in the 4.X branch of
Clang/LLVM (I reported it here
https://bugs.llvm.org/show_bug.cgi?id=36723).

This commit updates the README accordingly.

vrancurel/ntl,vrancurel/ntl,vrancurel/ntl