gcc8 -Wextra prints a warning when reinterpret_cast is used for conversion
between different types of function pointers. Avoid that by adding
Glib::bitwise_equivalent_cast<>() with a union with members of
the two types of function pointers.
* glib/src/optiongroup.ccg: Use Glib::bitwise_equivalent_cast<>() to convert
from a function pointer to void*.