appveyor: use MSYS2 shaderc package

There's a shaderc package in MSYS2 now. Using it should shave ten
minutes off the appveyor build.

TOOLS/appveyor-install.sh

@@ -23,6 +23,7 @@ pacman -S --noconfirm --needed \
     $MINGW_PACKAGE_PREFIX-lua51 \
     $MINGW_PACKAGE_PREFIX-ninja \
     $MINGW_PACKAGE_PREFIX-rubberband \
+    $MINGW_PACKAGE_PREFIX-shaderc \
     $MINGW_PACKAGE_PREFIX-uchardet \
     $MINGW_PACKAGE_PREFIX-vulkan
 
@@ -51,17 +52,6 @@ pacman -Sc --noconfirm
     make -j4 install
 )
 
-# Compile shaderc
-(
-    git clone --depth=1 https://github.com/google/shaderc && cd shaderc
-    "$PYTHON" utils/git-sync-deps
-
-    mkdir build && cd build
-    cmake -GNinja -DCMAKE_BUILD_TYPE=Release -DSHADERC_SKIP_TESTS=ON \
-          -DCMAKE_INSTALL_PREFIX=$MINGW_PREFIX ..
-    ninja install
-)
-
 # Compile SPIRV-Cross
 (
     git clone --depth=1 https://github.com/KhronosGroup/SPIRV-Cross && cd SPIRV-Cross