MSVC treats static_assert as a keyword in C mode and they implement in
the C++11 way. It does not support _Static_assert in C mode either.
This is a non-conforming extension (the keyword is outside the
implementer's namespace), so it is placed under -fms-compatibility
instead of -fms-extensions like most MSVC-specific keyword extensions.
Patch by Andrey Bokhanko!