Move check for unspecified CFLAGS in configure.ac

Move check for unspecified CFLAGS in configure.ac

configure.ac provides a check to see if the user has
specified CFLAGS and if not, it sets CFLAGS to a blank
(not NULL) string so that the resultant configure script
does not set '-g' and/or '-O2' by default. This check
occurs after AC_USE_SYSTEM_EXTENSIONS in the configure.ac
file. However, on at least some systems, such as OpenBSD,
AC_USE_SYSTEM_EXTENSIONS expands to include the code that
configure uses to set '-g -O2' so the check has no effect
and '-g' or '-O2' can not be turned off. This patch moves
the "CFLAGS specified" check so that it precedes the
AC_USE_SYSTEM_EXTENSIONS directive, in which case
everything works.

Change-Id: I68ed5a3c5129a55f7dcd7413c93f35cc7cb14b9c
Reviewed-on: http://gerrit.openafs.org/3688
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>