Makefile: avoid using -Werror with clang for now.

Apparently the default compiler on OS X and FreeBSD is now clang, and
it doesn't like our CHECK_VALUE_FITS() definition.  So for now, just
don't specify -Werror when we detect clang.

Thanks to Alexander Barton <alex@barton.de>, Thomas Klausner
<tk@giga.or.at>, Sebastian Schumb <sebastian@sebastians-site.de>, and
Zoran Zaric <zz@zoranzaric.de> for reporting the problem.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Alexander Barton <alex@barton.de>
Only allow "index --clear" to clear the default index (BUP_DIR/bupindex).

For now, disallow clearing "external" indexes.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>