Issue #6982: Add generated Lib/lib2to3/*.pickle files to 'make clean' target.

Thanks egreen.

Makefile.pre.in

@@ -1151,7 +1151,7 @@ TAGS::
 	for i in $(SRCDIRS); do etags -a $$i/*.[ch]; done
 
 # Sanitation targets -- clean leaves libraries, executables and tags
-# files, which clobber removes those as well
+# files, which clobber removes as well
 pycremoval:
 	find $(srcdir) -name '*.py[co]' -exec rm -f {} ';'
 
@@ -1160,6 +1160,7 @@ clean: pycremoval
 	find . -name '*.s[ol]' -exec rm -f {} ';'
 	find build -name 'fficonfig.h' -exec rm -f {} ';' || true
 	find build -name 'fficonfig.py' -exec rm -f {} ';' || true
+	-rm -f Lib/lib2to3/*Grammar*.pickle
 
 profile-removal:
 	find . -name '*.gc??' -exec rm -f {} ';'