Null and Undefined Macros
Both null and undefined macros expand to null strings, but a macro defined as a null string is considered defined in preprocessing expressions. To define a macro as a null string, specify no characters except spaces or tabs after the equal sign (=) in a command line or command file, and enclose the null string or definition in double quotation marks (" "). To undefine a macro, use !UNDEF. For more information, see Makefile Preprocessing Directives.