-Uname

   Removes (undefines) any initial definition of a symbol name as
   defined by -Dname.  You can use this method to remove a symbol name
   instead of using #undef in the source code.  You can use more than
   one -Uname argument on the command line.  This argument has no
   effect if you use the -no_cpp argument.  If you define and undefine
   a name on the same command line, undefining takes precedence.