don't fetch native Windows binaries on Unix

Makefile

@@ -310,6 +310,9 @@ distclean: clean
 
 dependencies: cli-dependencies native-dependencies
 
+windows-dependencies:
+	cd thirdparty && ./fetch-thirdparty-deps-windows.sh && cd ..
+
 cli-dependencies:
 	cd thirdparty && ./fetch-thirdparty-deps.sh && cd ..
 	@ $(CP_R) thirdparty/*.dll .

thirdparty/fetch-thirdparty-deps-windows.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+
+# Die on any error for Travis CI to automatically retry:
+set -e
+
+if [ ! -f windows/SDL2.dll ]; then
+	echo "Fetching SDL2 from nuget"
+	nuget install sdl2 -Version 2.0.3
+	cp ./sdl2.redist.2.0.3/build/native/bin/Win32/dynamic/SDL2.dll ./windows/
+	rm -rf sdl2.2.0.3 sdl2.redist.2.0.3
+fi
+
+if [ ! -f windows/freetype6.dll ]; then
+	echo "Fetching SharpFont from nuget"
+	nuget install SharpFont -Version 2.5.0.1
+	cp ./SharpFont.2.5.0.1/Content/freetype6.dll ./windows/
+	rm -rf SharpFont.2.5.0.1
+fi

thirdparty/fetch-thirdparty-deps.sh

@@ -35,7 +35,6 @@ if [ ! -f SharpFont.dll ]; then
 	nuget install SharpFont -Version 2.5.0.1
 	cp ./SharpFont.2.5.0.1/lib/net20/SharpFont* .
 	cp ./SharpFont.2.5.0.1/Content/SharpFont.dll.config .
-	cp ./SharpFont.2.5.0.1/Content/freetype6.dll ./windows/
 	rm -rf SharpFont.2.5.0.1
 fi
 
@@ -46,13 +45,6 @@ if [ ! -f nunit.framework.dll ]; then
 	rm -rf NUnit.2.6.4
 fi
 
-if [ ! -f windows/SDL2.dll ]; then
-	echo "Fetching SDL2 from nuget"
-	nuget install sdl2 -Version 2.0.3
-	cp ./sdl2.redist.2.0.3/build/native/bin/Win32/dynamic/SDL2.dll ./windows/
-	rm -rf sdl2.2.0.3 sdl2.redist.2.0.3
-fi
-
 if [ ! -f Mono.Nat.dll ]; then
 	echo "Fetching Mono.Nat from nuget"
 	nuget install Mono.Nat -Version 1.2.21