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

New in version 3.14: list of options to pass to link command.

CMAKE_REQUIRED_LIBRARIES
list of libraries to link.
CMAKE_REQUIRED_QUIET

New in version 3.1: execute quietly without messages.