[CMake] per configuration settings

[Dekeyser, Kris]

> I do not think there is a cross-platform way to do this right now. 
> However, "NDEBUG" is defined for all release builds and not 
> defined for 
> debug builds on all platforms.  You can use this in a header file to 
> define/undefine whatever macros you want.

Tip: with VS6 you can automatically include that header file with the /FI
compiler option without changing any code. The header file could be
generated by CMake if you prefer.

- Kris
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