[CMake] C# support again

[Joshua Jensen]

Pau Garcia i Quiles wrote:
> The developers of Qyoto (the C# binding to Qt) have written a 
> FindMono.cmake which might be useful:
> http://websvn.kde.org/trunk/KDE/kdebindings/cmake/modules/FindMono.cmake
This was useful, but I never really made it work well.

* ${CMAKE_CURRENT_SOURCE_DIR}/ must be prepended to every file in the 
sources list.
* All sources must have their forward slashes converted to backslashes.
* The extension .exe is complained about during nmake steps.  (NMAKE : 
fatal error U1073: don't know how to make 'hello.exe')
* It isn't integrated into ADD_EXECUTABLE().  That would be nice.
* It seems to require ALL to be added to the custom target.  I believe 
this makes the target build every time.

For me, it showed me how to add custom commands.  That's something I 
haven't done a lot of lately.

Thanks!  It was a better start than I had!

Josh