make FastCGI work on OS X

The thrift/proxygen patches are not done in their own repos due to:
- fbthrift has shifted towards dev in 'cpp2' while we're using 'cpp'.
- The copy of fbthrift in HHVM has already been modified.
- Sara kinda told me I could do it.

At some point we'll have to try to land these elsewhere and use them as
a submodule. Today is not the time.