if they are signed or unsigned.

Range-check enums is sound programming practice, but that concept
seems to be beyond the imagination of certain compiler people:

   vhp_decode.c:96:2: error: comparison of unsigned expression >= 0 is always true [-Werror=type-limits]

Add a dummy "MIN" value to the enum, give it value -1 to force
the compiler to use signed ints for the enum, and then check
that they're never negative.