OS X fixes

Summary: These commits allow compiling HHVM on OS X 10.9 with its native clang compiler.

This also requires https://github.com/facebook/hhvm/pull/3159, https://github.com/facebook/folly/issues/75 and https://github.com/hhvm/hhvm-third-party/issues/18.

CMake config:
```bash
cmake .. \
    -DLIBIBERTY_LIB=$(brew --prefix binutilsfb)/lib/x86_64/libiberty.a \
    -DLIBEVENT_LIB=$(brew --prefix libevent)/lib/libevent.dylib \
    -DLIBEVENT_INCLUDE_DIR=$(brew --prefix libevent)/include \
    -DICU_INCLUDE_DIR=$(brew --prefix icu4c)/include \
    -DICU_LIBRARY=$(brew --prefix icu4c)/lib/libicuuc.dylib \
    -DICU_I18N_LIBRARY=$(brew --prefix icu4c)/lib/libicui18n.dylib \
    -DICU_DATA_LIBRARY=$(brew --prefix icu4c)/lib/libicudata.dylib \
    -DREADLINE_INCLUDE_DIR=$(brew --prefix readline)/include \
    -DREADLINE_LIBRARY=$(brew --prefix readline)/lib/libreadline.dylib \
    -DCURL_INCLUDE_DIR=$(brew --prefix curl)/include \
    -DCURL_LIBRARY=$(brew --prefix curl)/lib/libcurl.dylib \
    -DBOOST_INCLUDE

Closes https://github.com/facebook/hhvm/pull/3182

Reviewed By: @ptarjan

Differential Revision: D1432897

Pulled By: svcscm