Check if the variable exists.

VAR      - the name of the variable
VARIABLE - variable to store the result

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