configure: Don't let autoconf pick our CFLAGS

configure: Don't let autoconf pick our CFLAGS

If the user hasn't specified CFLAGS on the command line to
./configure, then autoconf will set them to -g -O2 if the compiler
supports those options.

For compatibility with what OpenAFS has always done, and to let us
manually set optimisation and debugging flags later, disable this
behaviour.

Reviewed-on: http://gerrit.openafs.org/2895
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit b1f747c1f131fc1a9f8a5895fb06607a255a27d8)

Change-Id: Id35b288a56c8946a34b234eac1e899898d29ae65
Reviewed-on: http://gerrit.openafs.org/4003
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>