introspection: set PKG_CONFIG_PATH so that our in-tree libs come first when calling...

introspection: set PKG_CONFIG_PATH so that our in-tree libs come first when calling scanner

When calling gobject-introspection scanner, make sure our own
freshly-built libs within the source tree (well, build dir) come
first in the PKG_CONFIG_PATH. May or may not help to make sure
that it doesn't pick up older external plugins-base libs (or
.gir files) from outside the source tree / build directory as
dependencies of the introspected lib instead of using the
stuff we just built in a sibling directory.

https://bugzilla.gnome.org/show_bug.cgi?id=623698