package/pkg-meson.mk: explicitly specify pkg-config settings

package/pkg-meson.mk: explicitly specify pkg-config settings

meson is able to distinguish between host (= native) and target (=
cross) compilation. It will explicitly pass different options to
pkg-config to distinguish them. Therefore, we don't need to use the
pkg-config wrapper when using meson, and can instead pass the pkg-config
settings through the cross-compilation.conf.

This is important because in some situations (e.g. for the Python
configuration), meson sets the PKG_CONFIG_LIBDIR variable to a different
value before calling pkg-config. Relying on our wrapper script doesn't
work in that case (except if the script would unconditionally set
PKG_CONFIG_LIBDIR, which it doesn't do at the moment).

Add the sys_root and pkg_config_lib settings to cross-compilation.conf
and use pkgconf directly instead of the wrapper.

Note that this requires us to substitute STAGING_DIR as well, with an
absolute path. This is not a big deal since cross-compilation.conf is
regenerated for every package.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>