Makefile: improve nuget restore & clean

In some cases, nuget restore isn't sufficient to restore everything, especially if project.assets.json files
are missing or were removed. So we add a call to MSBuild -t:restore to build it.

Both nuget & msbuild are required because msbuild can only restore packages in 'packages.config' from version
16.5

.gitignore

@@ -7,7 +7,9 @@ PCtest
 bin
 obj
 
-allow_git_clean
+# Makefile-related files
+/allow_git_clean
+/stamp.nuget_*
 
 Resource.designer.cs
 R.java