vs2010: add explicit objects as 'Object' ItemGroup instead of link dependency

This has two effects:
1. It makes targets with only object files work (test case 88).
2. It adds the object files to the project in the VS IDE.

mesonbuild/backend/vs2010backend.py

@@ -492,9 +492,10 @@ class Vs2010Backend(backends.Backend):
             rel_path = self.relpath(lobj.subdir, target.subdir)
             linkname = os.path.join(rel_path, lobj.get_import_filename())
             additional_links.append(linkname)
+        additional_objects = []
         for o in self.flatten_object_list(target, down, include_dir_names=False):
             assert(isinstance(o, str))
-            additional_links.append(o)
+            additional_objects.append(o)
         if len(additional_links) > 0:
             additional_links.append('%(AdditionalDependencies)')
             ET.SubElement(link, 'AdditionalDependencies').text = ';'.join(additional_links)
@@ -549,13 +550,15 @@ class Vs2010Backend(backends.Backend):
                 # just the file name instead of the relative path to the file.
                 pch_file.text = os.path.split(header)[1]
 
-        if len(objects) > 0:
+        if len(objects) + len(additional_objects) > 0:
             # Do not add gen_objs to project file. Those are automatically used by MSBuild, because they are part of
             # the CustomBuildStep Outputs.
             inc_objs = ET.SubElement(root, 'ItemGroup')
             for s in objects:
                 relpath = s.rel_to_builddir(proj_to_src_root)
                 ET.SubElement(inc_objs, 'Object', Include=relpath)
+            for s in additional_objects:
+                ET.SubElement(inc_objs, 'Object', Include=s)
         ET.SubElement(root, 'Import', Project='$(VCTargetsPath)\Microsoft.Cpp.targets')
         # Reference the regen target.
         ig = ET.SubElement(root, 'ItemGroup')