packageconfig, add depends if feature is disabled

[Max Krummenacher <max.oss.09@gmail.com>]

Hi

The recent unconditional move towards gtk3 in oe-core forces me to
follow with some of my recipes.

I tried this with packageconfig, however there seems to be no easy way
to specify DEPENDS in case a packageconfig feature is disabled.
i.e. for gtk3 we need gtk+3 in depends while for gtk2 we need gtk+.
I ended up with:

    PACKAGECONFIG ?= "gtk3"
    PACKAGECONFIG[gtk3] = "--enable-gtk3,,gtk+3"
    python __anonymous () {
        depends = d.getVar("DEPENDS", d, 1)
        if 'gtk3' not in d.getVar('PACKAGECONFIG', True):
            d.setVar("DEPENDS", "%s gtk+" % depends)
    }

The following would also be possible but then nothing makes sure that
one, and exactly one of gtk2, gtk3 is in PACKAGECONFIG.

    PACKAGECONFIG ?= "gtk3"
    PACKAGECONFIG[gtk2] = ",,gtk+"
    PACKAGECONFIG[gtk3] = "--enable-gtk3,,gtk+3"

Any ideas?
Would this use case merit an extension to packageconfig, e.g.:

    # PACKAGECONFIG ??= "<default options>"
    # PACKAGECONFIG[foo] = "--enable-foo,--disable-foo,
                            foo_depends,foo_runtime_depends,
                            not_foo_depends,not_foo_rdepends"

Which would simplify the above to:

    PACKAGECONFIG ?= "gtk3"
    PACKAGECONFIG[gtk3] = "--enable-gtk3,,gtk+3,,gtk+"

Regards
Max