Removing MSVC C4800 from pragma block at the top of pybind11.h (#3141)

* Adding PYBIND11_COMPAT_BOOL_CAST to appease MSVC 2015 warning C4800.

* Replacing PYBIND11_COMPAT_BOOL_CAST with simpler != 0

* Extra parentheses (almost all compilers failed without these).
5 files changed