Add CMake support for using clang++ as the C++ compiler

There was a weird issue where CHECK_CXX_SOURCE_COMPILES wouldn't find
clock_gettime under clang++, switching to CHECK_FUNCTION_EXISTS worked
both under clang++ and g++ on Ubuntu 13.10
Merge pull request #1520 from alexmalyshev/master

Add CMake support for using clang++ as the C++ compiler
Add HNI support for the HipHopSpecific flag
Change ext/reflection from IDL to HNI
Change hphp_invoke_method back to accepting a CVarRef to better handle nulls
The $params passed to hphp_create_object can be null
Exit early if coercing fails for arguments on native functions
A warning is now generated due to incorrect argument type
Change back to using methInfo() and then handle HNI attributes separately
Use moduleInit, not moduleLoad
Add reflection to anchor-syms
Remove hphp_get_class_constant
Remove ext_reflection.h from idl include header