`WANT_IDN_PROTOTYPES` was necessary to avoid using a header that came
via an optional package. MS stopped distributing this package some
years ago and the winidn definitions are part of standard headers (via
`windows.h`) since Vista.
Auto-detect Vista inside `lib/idn_win32.c` and enable the manual
definitions if building for an older Windows.
This allows to delete this manual knob from all build-systems.
Also drop the `_SAL_VERSION` sub-case:
Our manual definitions are now only enabled with old systems. We assume
that code analysis is not run on such systems, allowing us to delete the
SAL-friendly flavour of these.
Reviewed-by: Jay Satiro
Closes#9793
b51560b9ff