Undefine preprocessor macro name.
This option applies only to source files that invoke the fpp or cpp preprocessor. It removes any initial definition of the preprocessor macro name created by -Dname on the same command line, including those implicitly placed there by the command-line driver, regardless of the order the options appear. It has no effect on any macro definitions in source files. Multiple -Uname flags can appear on the command line. There must be no space between -U and the macro name.