New "make" completion

[Michael Hwang <nomex45@xxxxxxxxx>]

Hello all,

I spent 3 frustrating days on a computer science lab,
all due to the fact that I mistyped a variable name. Instead of running
"make SIM=../seq/ssim", I was running "make SEQ=../seq/ssim". Of
course, make didn't complain, so I thought my coding was bad. Turns out
that wasn't the case.

To keep myself (or anyone else) from
making such a mistake again, I've added the ability to complete
variable names for make. They are presented alongside possible targets.
The file is entirely re-written, so I've provided it instead of a diff.
The completion should perform exactly the same as the old completion,
except possibly when including other makefiles.

A TODO: Variable completion should be sensitive to targets given to make. That
is, variables should only be presented if they are used in given
targets and their dependencies.

Michael Hwang