Draft: OpenWatcom: Improve support for compiling Windows resources

This is not yet finished due to two major issues:

• OpenWatcom doesn't support linking resources using wlink - instead, the normal process is to link the executable without the resources included, and then call wrc to attach the resources to the final executable. (https://github.com/open-watcom/open-watcom-v2/issues/748)
• wrc does not include the source directory by default. (https://github.com/open-watcom/open-watcom-v2/issues/747)

It might be possible to work around these issues from within CMake, but I'm not sure how to approach this.