Update CMakeLists.txt to prevent source tree corruption when building

[khobaib529]

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Prevent In-Source Builds

To prevent source tree corruption, this project does not allow in-source builds. The CMakeLists.txt file has been updated to enforce this restriction.

Behavior

If you attempt to build in the source directory, you will see the following error message:

CMake Error at CMakeLists.txt:5 (message):

FATAL: In-source builds are not allowed. You should create a separate directory for build files.