package/libgtk3: don't use pkgconf directly for host build

package/libgtk3: don't use pkgconf directly for host build

The build of host-libgtk3 calls $(HOST_DIR)/bin/pkgconf directly,
assuming that it will return correct results when building host
tools. It did work in practice without per-package directories, but is
not how pkg-config is used for host build in general: we recommend to
use $(HOST_DIR)/bin/pkg-config and we have in $(HOST_MAKE_ENV) a
number of environment variables that tell pkg-config to return results
relevant for host builds.

With per-package directories, calling $(HOST_DIR)/bin/pkgconf fails
badly, because it searches for .pc files in the per-package directory
of host-pkgconf itself, which obviously is empty.

So, we switch to using $(HOST_MAKE_ENV) $(PKG_CONFIG_HOST_BINARY),
which uses the regular pkg-config with the right environment
variables.

This allows the build of host-libgtk3 to find gdk-pixbuf-2.0 and
gio-2.0 built for the host, even in the context of
BR2_PER_PACKAGE_DIRECTORIES=y.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>