Make --update-cache work better

Due to an oversight, combining the --update-cache option with
explicitly specified modules was leading to an attempt to
open the --update-cache.so module - with predictably disappointing
results. Make this work as intended, and as described
in the man page.
build/win32/vs10/gengir.vcxproj: Add PlatformToolset tag

This makes it easier to upgrade to Visual Studio 2012/2013 formats,
like the other *.vcxproj files in GDK-Pixbuf.