[CMake] Easy target to create an object file?

[Tanner Lovelace]

For various reasons, on the windows platform, in addition to
creating a static library, I also need to create an object
file from one of the same source files, but with a different
set of #defines applied to it.  Is there any easier way to
do this other than just creating a custom target to do it?

Thanks much,
Tanner Lovelace