5.21 undefine option (-U)

This is the "definition list removal pattern" option.

This option has some usage constraints. It:

• may appear an unlimited number of times.
• may not be preset with environment variables or in initialization (rc) files.
• is a member of the define class of options.

Just like 'C', AutoGen uses #ifdef/#ifndef preprocessing directives. This option will cause the matching names to be removed from the list of defined values.