UWP builds should set PKG_CONFIG_PATH

[Manishearth]

UWP builds use a local gstreamer from msvc-dependencies. They should override the existing PKG_CONFIG_PATH during the build so that the build doesn't go looking for x86 binaries

I suspect this is why my older build was failing at dynamic link time, I had PKG_CONFIG_PATH set to x86 gstreamer, and uninstalling gstreamer fixed the problem but now the build fails when it builds glib-sys since it can't find glib. I'm not sure how it worked before since it was a clean build the first time, but it's definitely broken now. (My temporary fix was to just set PKG_CONFIG_PATH globally, it may be worth setting the GSTREAMER_1_0_ROOT variables instead)

cc @jdm

[Manishearth]

Actually, this is more complicated because servo refuses to build without gstreamer_webrtc (we use some types from that crate) even though UWP builds don't have webrtc enabled anyway.

[Manishearth]

I'm surprised anything worked at all here, it seems like for a successful cross UWP servo build you have to have a host PKG_CONFIG_PATH that just makes the build work and then the DLLs get pulled in later. Except it seems like that was causing build problems last week.

[Manishearth]

Adding a fake gstreamer-webrtc-1.0.pc file to the gstreamer-uwp msvc-dependencies folder allows the build to work (confirmed that it runs), and this should be a decent stopgap until we get an actual gstwebrtc for UWP