Nowadays ~/.icons is not used anymore as the preferred location for custom
user icon themes; XDG_DATA_HOME/icons (aka ~/.local/share/icons) is what
toolkits like GTK prefer. Prepend that location to the default xcursor path, so
that cursor themes installed there can be used by apps and toolkits that use
libXcursor.
Port of https://cgit.freedesktop.org/xorg/lib/libXcursor/commit/src?id=2263c196cb0dcb8547b378df7b35f83b8b99c01e