variable_watch
Watch the CMake variable for change.
variable_watch(<variable> [<command>])
If the specified <variable>
changes, a message will be printed to inform about the change.
Additionally, if <command>
is given, this command will be executed. The command will receive the following arguments: COMMAND(<variable> <access> <value> <current_list_file> <stack>)
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Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.17/command/variable_watch.html