The contents of the macro directory are too complicated for git.mk to
handle: the contents are filled by autotools and may change between
systems or autotools releases; and we additionally ship our own m4
macros.
To avoid unnecessary noise on the `git status` output we should simply
add an ignore file specifically for the macro directory.
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
