A fundamental problem with the configure_file() command is CMake is that
the author of the template file config.h.in must know which variables
all modules need to have defined, and this is not easily communicated.
The idea behind this function is: Each Find-module can return a list of
variables that it has filled with probed values, and that should be
written to config.h, which is again included by the header file of this
module.
Each project thus defines its own config.h file, which is common between
all module's headers.
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