cmake: Add support for builtin codegen target

Additionally, this change removes unnecessary braces in the `if()`
command for improved robustness, readability and consistency with CMake
guidelines.

CMakeLists.txt

@@ -9,10 +9,16 @@
 #  - CMake 3.26.5, https://mirror.stream.centos.org/9-stream/AppStream/x86_64/os/Packages/
 cmake_minimum_required(VERSION 3.22)
 
-if (${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_BINARY_DIR})
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR)
   message(FATAL_ERROR "In-source builds are not allowed.")
 endif()
 
+if(POLICY CMP0171)
+  # `codegen` is a reserved target name.
+  # See: https://cmake.org/cmake/help/latest/policy/CMP0171.html
+  cmake_policy(SET CMP0171 NEW)
+endif()
+
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 # Project / Package metadata
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