[CMake] How to exclude some files from building?

[kipade]

There are some files I do not want to build if the specified condition was ture.
For example, for testing, I want build a new file witch include a main entry just
for testing; if not, a new file would be compile for normal task.
I do not want to write two main entries in the same file just using macros to
control compiling. I want to use different clean and simple files.
So, how to do it?
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