[CMake] Change variable default

[Ilja Golshtein]

Hello!

There are some CMAKE variables with default values. For example CMAKE_CXX_FLAGS_DEBUG default value is '-g'. 
How is it possible to make it '-g -Wall'?

What I don't want to have: User choice overwritten.

What I want to have: Modified a variable cmake-time default. It must be possible to change the variable via cmake -D or ccmake GUI.

Thanks.

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Best regards,
Ilja Golshtein.