The recent change to the enum declaration for GtkCssChange actually
relied on compiler-dependent behavior, which also breaks the build on
some non-GCC compilers, such as Visual Studio.  As noted in the
G_STATIC_ASSERT line just beneath this declaration, we need to change
this enum declaration to #define's, in order to fix the build in such
situations.

https://bugzilla.gnome.org/show_bug.cgi?id=752814