Fix compilation for libc++ 3.9

Fix compilation for libc++ 3.9

Summary:
libc++ 3.9 and above define the __throw* functions inside stdexcept, so
defining them ourselves leads to compilation errors when compiling
against libc++ 3.9. Add appropriate preprocessor guards to avoid this.

Reviewed By: meyering

Differential Revision: D3898284

fbshipit-source-id: 435a28c2b3a83ee4d8f5af0df0343c524469011e