Bug 1359852 - Use REPORT_BUILD for .xpt files; r=chmanchester

By using $(REPORT_BUILD) instead of just echoing the filename, there is
no change during a regular build without REBUILD_CHECK, but specifying
REBUILD_CHECK in the environment will show which files triggered an .xpt
to rebuild. This is helpful in debugging why these files may be built
unnecessarily.

MozReview-Commit-ID: GGNaKAl02Ea

--HG--
extra : rebase_source : 098a81265deed9afc0b284d9863f18ebd74fda33

config/makefiles/xpidl/Makefile.in

@@ -33,7 +33,7 @@ process_py := $(topsrcdir)/python/mozbuild/mozbuild/action/xpidl-process.py
 # TODO we should use py_action, but that would require extra directories to be
 # in the virtualenv.
 %.xpt:
-	@echo "$(@F)"
+	$(REPORT_BUILD)
 	$(PYTHON_PATH) $(PLY_INCLUDE) -I$(topsrcdir)/xpcom/idl-parser -I$(DEPTH)/xpcom/idl-parser/xpidl \
 		$(process_py) --cache-dir $(DEPTH)/xpcom/idl-parser/xpidl --depsdir $(idl_deps_dir) \
 		$(dist_idl_dir) $(dist_include_dir) $(@D) $(libxul_sdk_includes) \