buildsys: skip compiler flag checking for clean targets

buildsys: skip compiler flag checking for clean targets

As suggested by Bernhard, there is no point in evaluating the compiler's
flag availability when cleaning, so skip things in that case.  If there
are variables that change targets based on the flags, then things are
already broken and need fixing independently.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>