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  5.42 Incomplete enum Types

   You can define an enum tag without specifying its possible values. This results
   in an incomplete type, much like what you get if you write struct foo without
   describing the elements. A later declaration which does specify the possible
   values completes the type.

   You can't allocate variables or storage using the type while it is incomplete.
   However, you can work with pointers to that type.

   This extension may not be very useful, but it makes the handling of enum more
   consistent with the way struct and union are handled.

   This extension is not supported by GNU C++.

References

   1. https://gcc.gnu.org/onlinedocs/gcc-4.3.0/gcc/index.html#Top
   2. https://gcc.gnu.org/onlinedocs/gcc-4.3.0/gcc/C-Extensions.html#C-Extensions
   3. https://gcc.gnu.org/onlinedocs/gcc-4.3.0/gcc/Alternate-Keywords.html#Alternate-Keywords
   4. https://gcc.gnu.org/onlinedocs/gcc-4.3.0/gcc/Function-Names.html#Function-Names
   5. https://gcc.gnu.org/onlinedocs/gcc-4.3.0/gcc/Function-Names.html#Function-Names
   6. https://gcc.gnu.org/onlinedocs/gcc-4.3.0/gcc/Alternate-Keywords.html#Alternate-Keywords
   7. https://gcc.gnu.org/onlinedocs/gcc-4.3.0/gcc/C-Extensions.html#C-Extensions