gcc8 -Wextra prints a warning when a single reinterpret_cast is used for
conversion between different types of function pointers. The previous fix
with a union in Glib::bitwise_equivalent_cast<>() is not standard C++.
Rename the function to Glib::function_pointer_cast<>(), and use two
reinterpret_casts as recommended in gcc's documentation.

* glib/src/optiongroup.ccg: Use a reinterpret_cast to convert from
a function pointer to void*. That's possible now. It's "conditionally
supported", starting with C++11.

See https://github.com/libsigcplusplus/libsigcplusplus/issues/8