Only specify CFLAGS_NO* with --enable-checking

Only specify CFLAGS_NO* with --enable-checking

Currently we are always specifying e.g. CFLAGS_NOUNUSED as -Wno-unused
when we think we are compiling with gcc. Since autoconf always tries
to use gcc when possible, this breaks the build if we have a gcc
available but we use another cc for building.

This should be fixed otherwise, but in the meantime at least make this
only happen if warnings are actually turned on, so the build is less
likely to break with a default ./configure invocation.

Reviewed-on: http://gerrit.openafs.org/3366
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 18793d4bfaeb88fd456c8d489165139126c0d293)

Change-Id: Ifd31548dfecdd1a4838047e2548a8b2265422d26
Reviewed-on: http://gerrit.openafs.org/3557
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>