New _make checked in

[Wayne Davison <wayned@xxxxxxxxxxxxxxxxxxxxx>]

I've committed the _make completer that was mentioned in 20277 with a
couple changes:

 - The code that sets variables in the environment is safer, and will
   no longer fail with an error if a makefile has something like
   FOO!=bar in it (though it does not yet understand the extended !=
   or ?= assignment styles that Geoff mentioned).

 - One eval was replaced with the more optimal code that Bart suggested.

..wayne..