Portable way to copy va_list variables.
In order to use this function, you must include string.h yourself,
because this macro may use
memmove() and GLib does not include
string.h for you.
Each invocation of
G_VA_COPY (ap1, ap2) must be matched with a
va_end (ap1) call in the same function.
This function is not directly available to language bindings.