CMAKE__FLAGS_
From Get docs
Cmake/docs/3.21/variable/cmake lang flags config
CMAKE__FLAGS_
Flags for language <LANG> when building for the <CONFIG> configuration.
The flags in this variable will be passed to the compiler after those in the CMAKE__FLAGS variable, but before flags added by the add_compile_options() or target_compile_options() commands.
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Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.21/variable/CMAKE_LANG_FLAGS_CONFIG.html
