Our build files were relying 'sysroot' to find gexiv2.h but this is
not possible with Apple Clang om which sysroot points to macOS SDK.
So, exotic environments like Homebrew were failing. Let's fix this.
We pass 2 GeglColor through the wire now. Since it is passed very early
(when sharing the configuration), I had some issues with initialization
order of GEGL, and in particular when calling gegl_init() before
gegl_config() inside _gimp_config(), I had a bunch of such criticals:
> Plugin script-fu: GLib-GObject: CRITICAL: Two different plugins tried to register 'GeglOpPlugIn-transform-core'
Anyway in the end, I store the passed colors as raw bytes and strings in
the GPConfig object, and re-construct the GeglColor last minute in
_gimp_config().
