bpo-30492: Allow make clinic to work out of tree. (#1836)

* bpo-30492: Allow make clinic to work out of tree.

* Use os.curdir instead of "." as the default value.
diff --git a/Makefile.pre.in b/Makefile.pre.in
index 0aefcf2..ee1cd4a 100644
--- a/Makefile.pre.in
+++ b/Makefile.pre.in
@@ -528,7 +528,7 @@
 # (depends on python having already been built)
 .PHONY=clinic
 clinic: $(BUILDPYTHON) $(srcdir)/Modules/_blake2/blake2s_impl.c
-	$(RUNSHARED) $(PYTHON_FOR_BUILD) ./Tools/clinic/clinic.py --make
+	$(RUNSHARED) $(PYTHON_FOR_BUILD) $(srcdir)/Tools/clinic/clinic.py --make --srcdir $(srcdir)
 
 # Build the interpreter
 $(BUILDPYTHON):	Programs/python.o $(LIBRARY) $(LDLIBRARY) $(PY3LIBRARY)