vs2010: fix include directories

Everything in the VS project file is relative to the project file itself.
The project file gets put in the target.subdir, so to include files from
there we just need to use '.'. To include from the private dir, we
need to use the relative path from the target dir to the target private
dir.

mesonbuild/backend/vs2010backend.py

@@ -421,11 +421,8 @@ class Vs2010Backend(backends.Backend):
         clconf = ET.SubElement(compiles, 'ClCompile')
         opt = ET.SubElement(clconf, 'Optimization')
         opt.text = 'disabled'
-        inc_dirs = [proj_to_src_dir, self.get_target_private_dir(target)]
-        cur_dir = target.subdir
-        if cur_dir == '':
-            cur_dir= '.'
-        inc_dirs.append(cur_dir)
+        inc_dirs = ['.', self.relpath(self.get_target_private_dir(target), self.get_target_dir(target)),
+                    proj_to_src_dir]
 
         extra_args = {'c': [], 'cpp': []}
         for l, args in self.environment.coredata.external_args.items():