Now that we assume gcc 7.5 as a minimum, we have the option
of changing to a newer C standard. The two major ones that
I think apply are _Generic and _Static_assert.
While Paolo created a remarkably functional replacement for _Generic
using builtins, the error messages that you get out of the keyword
are*vastly* more intelligable, and the syntax is easier to read.
While I'd like to prefer _Static_assert over QEMU_BUILD_BUG_ON
going forward, and would like to convert existing uses, that is
a much bigger job. Especially since the test condition is inverted.
In the meantime, can drop the configure detection.