# CheckVariableExists

Check if the variable exists.

`CHECK_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_LINK_OPTIONS = list of options to pass to link command
    CMAKE_REQUIRED_LIBRARIES = list of libraries to link
    CMAKE_REQUIRED_QUIET = execute quietly without messages