Bugfix update makeprg on settings changes

Every time a Makefile settings is changed, if a make command has been already
defined, the makeprg VIM settings is updated to use the new values.

Signed-off-by: Patrick Bellasi <derkling@gmail.com>
Keep track of previous values on update

If a Makefile parameter is going to be updated, the current value for the
parameter is proposed as default.

Signed-off-by: Patrick Bellasi <derkling@gmail.com>