Handle lack of <bits/c++config.h> and <bits/functexcept.h>

Handle lack of <bits/c++config.h> and <bits/functexcept.h>

Summary:
- Clang's libc++ doesn't provide these header files. Detect libc++ via
the _LIBCPP_VERSION symbol (pulling it in by sourcing some header
files earlier if necessary) and avoid using these files.

Test Plan:
- fbconfig -r folly && fbmake runtests
- ./configure && make check on Ubuntu/FC/Mac

Reviewed By: andrewjcg@fb.com

FB internal diff: D998595