Simplify gitignore build directories [skip ci]

[bencoman]

I noticed that modified Makefiles weren't being pickup up by git status. It was too tedious trying to fix the existing definitions, so I've condensed them down to 4 lines from 34. I'm reasonably sure the result is the same, but if I've missed something its low impact, so maybe just integrate first to let everyone suck-n-see.
Doing a git status before each git commit will let you confirm that its working.

[bencoman]

Bah! HOLD for a minute

[krono]

@bencoman Are you interested in reviewing thas since the .gitignore changed?