Use shallow-cloned submodule for prebuilts

.gitmodules

@@ -13,3 +13,7 @@
 [submodule "h264bitstream/h264bitstream"]
 	path = h264bitstream/h264bitstream
 	url = https://github.com/aizvorski/h264bitstream.git
+[submodule "libs"]
+	path = libs
+	url = https://github.com/cgutman/moonlight-qt-prebuilts.git
+	shallow = true

appveyor.yml

@@ -32,7 +32,9 @@ install:
   - sh: 'if [[ "$BUILD_TARGET" = linux ]]; then mkdir $HOME/bin && wget -O $HOME/bin/linuxdeployqt https://github.com/probonopd/linuxdeployqt/releases/download/continuous/linuxdeployqt-continuous-x86_64.AppImage && chmod a+x $HOME/bin/linuxdeployqt; fi'
 
 before_build:
-  - 'git submodule update --init --recursive'
+  - 'git -c submodule.libs.update=none submodule update --init --recursive'
+  - cmd: 'git submodule update --init --recursive'
+  - sh: '[ "$BUILD_TARGET" != macos ] || git submodule update --init --recursive'
   - cmd: 'set OLDPATH=%PATH%'
   - cmd: 'set /p VERSION=<app\version.txt'
   - sh: 'export VERSION=`cat app/version.txt`'