Set env vars to let the build know where GStreamer libs are

appveyor.yml

@@ -43,13 +43,17 @@ cache:
   - .ccache
 
 install:
+  - choco install pkgconfiglite
   - appveyor-retry appveyor DownloadFile https://gstreamer.freedesktop.org/data/pkg/windows/1.14.1/gstreamer-1.0-devel-x86_64-1.14.1.msi
   - appveyor-retry appveyor DownloadFile https://gstreamer.freedesktop.org/data/pkg/windows/1.14.1/gstreamer-1.0-x86_64-1.14.1.msi
   - msiexec /i gstreamer-1.0-devel-x86_64-1.14.1.msi /quiet /qn /norestart /log install-devel.log
   - msiexec /i gstreamer-1.0-x86_64-1.14.1.msi /quiet /qn /norestart /log install.log
   - appveyor-retry appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
   - rustup-init.exe -y --default-host x86_64-pc-windows-msvc --default-toolchain none
   - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin
+  - set PKG_CONFIG_PATH=%PKG_CONFIG_PATH%;C:\gstreamer\1.0\x86_64\lib\pkgconfig
+  - set LIB=C:\gstreamer\1.0\x86_64\lib
+  - set LIBPATH=C:\gstreamer\1.0\x86_64\lib
   - rustup -V
   - mach rustc --version
   - mach cargo --version