MoltenVK: Don't run fetchDependencies unnecessarily

Externals/MoltenVK/CMakeLists.txt

@@ -6,7 +6,7 @@ ExternalProject_Add(MoltenVK
   GIT_REPOSITORY https://github.com/KhronosGroup/MoltenVK.git
   GIT_TAG ${MOLTENVK_VERSION}
 
-  CONFIGURE_COMMAND <SOURCE_DIR>/fetchDependencies --macos
+  CONFIGURE_COMMAND ${CMAKE_CURRENT_LIST_DIR}/configure.sh <LOG_DIR> <SOURCE_DIR> ${MOLTENVK_VERSION}
 
   BUILD_COMMAND make -C <SOURCE_DIR> macos
   BUILD_IN_SOURCE ON

Externals/MoltenVK/configure.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+
+# To lower build times, we avoid running the fetchDependencies script if the MoltenVK
+# version didn't change. The last-built MoltenVK version is stored inside a file in
+# the timestamp directory. If the file doesn't exist or the file contains a different
+# MoltenVK version, fetchDependencies is ran.
+#
+# Usage: configure.sh <timestamp directory> <source directory> <MoltenVK version>
+#
+
+set -e
+
+VERSION_PATH="$1/MoltenVK-last-version.txt"
+CURRENT_VERSION="$3"
+LAST_VERSION=$(cat "$VERSION_PATH" || true)
+
+if ! [ "$LAST_VERSION" = "$3" ]; then
+  $2/fetchDependencies --macos
+  echo $CURRENT_VERSION > $VERSION_PATH
+fi