Check if the include file exists.

INCLUDE  - name of include file
VARIABLE - variable to return result

An optional third argument is the CFlags to add to the compile line or you can use CMAKE_REQUIRED_FLAGS.

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_INCLUDES = list of include directories