Conceptually, these functions clearly ought to be fine for a const
structure. This avoids _G_TYPE_CVH (the implementation of
G_TYPE_CHECK_VALUE_TYPE, G_VALUE_HOLDS, G_VALUE_HOLDS_BOXED etc.)
needing to cast to a mutable GValue, which causes
G_VALUE_HOLDS (cv, type) to issue warnings under gcc -Wcast-qual if
cv is a const GValue *.
https://bugzilla.gnome.org/show_bug.cgi?id=734479