The integer size checks have been replaced with KWIML so their
hard-coded results are no longer needed.
Tell `TEST_BIG_ENDIAN` to use `unsigned short` as the 16-bit integer.
UNIX device APIs do not exist on Windows.
FreeBSD libmd does not exist on Windows.
Fix typo in existing `HAVE_READPASSPHRASE_H` setting.
Our bundled third-party libraries perform many `try_compile` checks for
compatibility with their upstream build systems. For many of the checks
we already know the result for compilers we support for building CMake
itself, especially on Windows. Hard-code known results to avoid running
the checks.
Brad King