configure: Restore saved CFLAGS

configure: Restore saved CFLAGS

When we test for whether the C compiler can take the
-fno-strength-reduce flag, we add the flag to CFLAGS to do so.
However, we were not restoring the old value of this flag when we
completed the test, and so we were always setting -fno-strength-reduce
in the userspace compile.

Previously, this was harmless, as we always overwrote CFLAGS, but if
we're moving to a world where we honour the user's setting of CFLAGS,
we need to not leak changes in this way.

Reviewed-on: http://gerrit.openafs.org/2894
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 907521feb1b9c7a6c816866a672c7ab84dac6abd)

Change-Id: Ie8c2a80b204754f1c17d9fa5e06903a7cdf7e4d2
Reviewed-on: http://gerrit.openafs.org/4004
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>