This add correct Open Watcom support for 16-bit Windows 3.x.
It replace existing strange mixture with WIN32 stuff which implement 16-bit Windows target partially as part of WIN32 stuff.
Now pre-defined OS ID Windows3x is used instead of confusing WIN32.
It support properly 16-bit and 32-bit application for 16-bit Windows host.
32-bit applications are build with OW WIN386 extender.

It is used similar as for other platforms by set CMAKE_SYSTEM_NAME=Windows3x and CMAKE_SYSTEM_PROCESSOR=I86 for 16-bit application or CMAKE_SYSTEM_PROCESSOR=x86 for 32-bit WIN386 extender application running on 16-bit Windows 3.x.
CMAKE_SYSTEM_NAME=Windows is used only for WIN32 applications.