How to tell CMake which libpng.dylib to use?

[Jeremy Lavergne]

> setting CMake
> -D flags in the PortGroup files is about as good as we can do, then
> setting specific variables on a port-by-port basis.  At least IMHO and
> experience; that said, I'm certainly open to suggestions by others. -

I recall we had shoved many of these flags into the KDE PortGroup.

Since they're universal and it is not an error to define -D flags that are not used, perhaps we should have a proc or variable in Cmake's PortGroup which defines any variables that we come across. Then by using the cmake PortGroup a port will have these available.