configure: Error if knc driver is requested, but linux/i2c-dev.h is missing or not from i2c-tools

configure.ac

@@ -391,7 +391,9 @@ AC_ARG_ENABLE([knc],
 if test "x$knc" = xyes; then
 	AC_CHECK_HEADERS([linux/i2c-dev-user.h])
 	AC_CHECK_DECL([i2c_smbus_read_word_data],[true],[
+		AC_MSG_ERROR([linux/i2c-dev.h header from i2c-tools (NOT linux headers) is required for knc driver])
 	],[
+		#include <stddef.h>
 		#ifdef HAVE_LINUX_I2C_DEV_USER_H
 		#include <linux/i2c-dev-user.h>
 		#else