Fix 4.01 compilers on MacOS X

For system compilers, do "brew update && brew upgrade" to get the 4.01.0_1 revision which works.

If you have a locally compiled switch that breaks on XCode 5.1, do "opam update && opam switch reinstall 4.01.0" and it should pick up these changes.

Closes #1787 
Closes #1786