Portable StaticAssertExpr

The current implementation of StaticAssertExpr() uses the GCC extension
statement expressions.

In this patch, I'm proposing a different implementation that doesn't
require nonstandard extensions. The trick is to put the static_assert()
into a struct definition. This appears to be a common way to do this.
(See [0] for references.)

Unfortunately, MSVC before version 19.33 fails to compile this, but
since it was fixed later, I think this was just a bug that was later
fixed. I'm keeping the old workaround code as a fallback for this case,
but eventually we can remove that.

For C++, the struct trick doesn't work, but I found a different
implementation that works on all supported compilers, using lambda
expressions. I found this at [1].

[0]: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3715.pdf
[1]: https://stackoverflow.com/questions/31311748