MSVC Builds: Update Introspection Build Process

GObject-Introspection was recently changed to support acquiring the name of
the DLL from a library (.lib, etc) that was passed into g-ir-scanner on
Windows, like the *nix builds, instead of directly passing in the name of
the DLL.

This updates the introspection build process, so that introspection files
for Pango can continue to be properly built.