Windows port: compute GOBJECT_INTROSPECTION_LIBDIR at runtime.

Windows port: compute GOBJECT_INTROSPECTION_LIBDIR at runtime.

Otherwise, we fail to properly locate the typelibs, because on Windows
the value of GOBJECT_INTROSPECTION_LIBDIR depends on where Glib has been
installed. Due to the nature of how we handle software that depends on
Glib on Windows (it is recommended that each program bundles it's private
copy), we're working in a "multi-prefixed" environment. Hence the value
computed at build time will most likely not even exist at runtime.

https://bugzilla.gnome.org/show_bug.cgi?id=620566