build: support cross-compilation by natively building gtk-update-icon-cache

build: support cross-compilation by natively building gtk-update-icon-cache

When cross-compiling, instead of depending on a natively built GTK+ (which means
building Glib, ATK, Pango, gdk-pixbuf, libX11...) for gtk-update-icon-cache,
find the host compiler and gdk-pixbuf, and build another gtk-update-icon-cache
with that.

This uses AX_PROG_CC_FOR_BUILD from autostars to find the host compiler, and
assumes that you'd set PKG_CONFIG_FOR_BUILD to a host pkg-config binary.

https://bugzilla.gnome.org/show_bug.cgi?id=691301