Add CMake support for Open Watcom

This (WIP) patch adds CMake support for the Open Watcom 16 bit toolchain (DOS16, Win16 and OS/2 16).
Steps to use:
1. Install OW with 16 bit targets
2. Extract the ZIP to ReactOS\modules\ros16bit
3. Copy wlsystem.lnk to %WATCOM%\binw
4. Start menu: Open Watcom C - C++ -> Build Environment
5. Add RosBE\Bin to PATH
6. In ros16bit run configure.cmd
7. cd output-OpenWatcom-i86\ros16bit
8. Run build.bat