deps/Makefile fixes for GLPK

Fix distclean-glpk target to remove downloaded glpk archive
Fix `USE_SYSTEM_GLPK=1` to actually not download and build GLPK. (This has been happening on the last few Travis builds, for instance)

@vtjnash, could you take a look at this to make sure I'm doing things the "right" way?  You just updated a bunch of stuff in `deps/Makefile`, I want to make sure that I'm not mucking up the Makefile again with bad practices.