Fix folly on OSX and BSD in prep for FastCGI on HHVM

Fix folly on OSX and BSD in prep for FastCGI on HHVM

Summary:
A recent change in folly/MemoryMapping.cpp uses MAP_ANONYMOUS, which is
named MAP_ANON on OSX/BSD.

We need folly and third-party changes in order to land a pull request for HHVM
that starts to get FastCGI running on OSX.

See the checklist of the HHVM pull request here: https://github.com/facebook/hhvm/pull/2944#issuecomment-47281003

Closes #67

GitHub Author: Daniel Sloof <goapsychadelic@gmail.com>

@override-unit-failures

Test Plan: fbmake runtests 100%

Reviewed By: pt@fb.com, njormrod@fb.com

FB internal diff: D1407393