Change default value of build systems' `flags_from_environ` to `False`

[vkarak]

I think that the main use case and what the user would expect in first place is to simply run make, cmake etc. and not append the compiler definitions and flags from the environment.

[victorusu]

I agree with this. I think this is the right move. IMHO, That's the "normal behaviour". If one wants to have the definitions from the environment one should set flags_from_environ to True. I just wonder how many tests will break because they rely on this "feature"... 🤔

[vkarak]

Maybe the default should be per build system. For example, it makes sense for the default to True for the SingleSource build system and False for the others. @victorusu what do you think?

[vkarak]

Eventually, I think that the current behaviour is correct, because otherwise the build system could ignore completely the environment. So forcing users to set flags_from_environ = False is a way to explicitly state that I don't need the environment definitions for compilers and flags. I will add a note about this in the new tutorial.