Operator to read environment variables.
Use the syntax $ENV{VAR} to read environment variable VAR.
To test whether an environment variable is defined, use the signature if(DEFINED ENV{<name>}) of the if() command.
See the set() and unset() commands to see how to write or remove environment variables.
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Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.19/variable/ENV.html