Keep downloaded files in a separate directory

As a side effect, this re-enables building the Windows installer on Linux hosts.

thirdparty/fetch-thirdparty-deps-osx.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+
+download_dir="${0%/*}/download/osx"
+mkdir -p "$download_dir"
+cd "$download_dir"
+
+if [ ! -f libSDL2.dylib ]; then
+	echo "Fetching OS X SDL2 library from GitHub."
+	curl -LOs https://raw.githubusercontent.com/OpenRA/OpenRALauncherOSX/master/dependencies/libSDL2.dylib
+fi
+
+if [ ! -f liblua.5.1.dylib ]; then
+	echo "Fetching OS X Lua 5.1 library from GitHub."
+	curl -LOs https://raw.githubusercontent.com/OpenRA/OpenRALauncherOSX/master/dependencies/liblua.5.1.dylib
+fi
+
+if [ ! -f Eluant.dll.config ]; then
+	echo "Fetching OS X Lua configuration file from GitHub."
+	curl -LOs https://raw.githubusercontent.com/OpenRA/OpenRALauncherOSX/master/dependencies/Eluant.dll.config
+fi