configure: disallow ccache during compile tests

If the user is using ccache during the configuration step,
it may interfere with some of the configuration tests,
particularly the "Is ccache interfering with macro analysis" step,
which is a bit of a poetic problem.

1) Disallow ccache from reading from the cache during configure,
   but don't disable it entirely to allow us to see if it causes other
   problems.

2) Force off CCACHE_CPP2 during the ccache test to get a deterministic
   answer over whether or not we need to enable that feature later.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <1446055000-29150-1-git-send-email-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>