package/meson: determine g-ir-scanner and g-ir-compiler paths from pkgconfig

Currently, meson hard codes the paths of these binaries which results in
cross-compiled environments to run the host versions of these tools.
However, GObject-introspection provides the appropriate paths to these
utilities via pkg-config

find_program is needed in the case g-i is built as a subproject. If
g-ir-scanner or g-ir-compiler are in the build or source directory use those.
If they aren't found in the source directory, use the results from pkg-config.

Backport two upstream commits to fix the issue.

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Tested-by: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>