use 'include' in lecture Makefiles.

- move common things into ../MakeSrc.mk
- put "include ../MakeSrc.mk" in each Makefile that need these
- easier to update and track the updates.
Remove Lectures/.gitignore; no longer used
Rename MakeSrc.mk -> MakeSrc_lectures.mk
Add comment to the top of MakeSrc_lectures.mk