[gtkmm] IOCondition holds illegal values

[Eric Buddington <eric ma-northadams1b-39 bur adelphia net>]

gtkmm-1.3.21.

My callback function tests for all six enumerations for
Glib::IOCondition.

In the case of a file descriptor that is a pipe to a dead child, I am
getting values '20' and '12', which match none of IO_IN, IO_OUT,
IO_PRI, IO_ERR, IO_HUP, or IO_NVAL.

I assume this is an error in the underlying libraries.... ?

-Eric