CheckVariableExists

Check if the variable exists.

CHECK_VARIABLE_EXISTS(VAR VARIABLE)
VAR      - the name of the variable
VARIABLE - variable to store the result
           Will be created as an internal cache variable.

This macro is only for C variables.

The following variables may be set before calling this macro to modify the way the check is run:

CMAKE_REQUIRED_FLAGS = string of compile command line flags
CMAKE_REQUIRED_DEFINITIONS = list of macros to define (-DFOO=bar)
CMAKE_REQUIRED_LIBRARIES = list of libraries to link
CMAKE_REQUIRED_QUIET = execute quietly without messages

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Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.6/module/CheckVariableExists.html