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.

Change-Id: Ic78d5f824433d94d76f16c107af3488132d57155
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>