autoconf: One CC to rule them all

autoconf: One CC to rule them all

(well, in userspace at least)

We have 3 different ways of specifying the C compiler - CC, CCOBJ and
MT_CC. On most platforms these are set to identical values by the
configure script. However, this causes problems for a user who wants
to override the default complier choice by doing
    make CC=my-favorite-compiler
as this doesn't catch all of the ways we specify the compiler [1]

So, change the specification of CCOBJ and MT_CC so that, by default,
they reference CC, rather than copying its value. This means that on
platforms where CC is the appropriate compiler to use in all situations,
a user need only change CC to modify their compiler choice.

[1] - it might be argued that the correct place to substitue the
      compiler is in configure's environment, which does currently work.
      However, this doesn't work with special things, like static
      analysers, that can build the source tree, but not the configure
      tests.

Change-Id: Ib95e2ac00b36f33c3cb2491feacd6c898f8f61dc
Reviewed-on: http://gerrit.openafs.org/7091
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>