PRIVATE_HEADER
Specify private header files in a FRAMEWORK shared library target.
Shared library targets marked with the FRAMEWORK property generate frameworks on macOS, iOS and normal shared libraries on other platforms. This property may be set to a list of header files to be placed in the PrivateHeaders directory inside the framework folder. On non-Apple platforms these headers may be installed using the PRIVATE_HEADER option to the install(TARGETS) command.
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Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.17/prop_tgt/PRIVATE_HEADER.html