autoconf orthogonality

It helps automated builds to be able to specify both --with and --without,
--enable and --disable.  This patch begins some of that for bulk_extractor's
autoconf.
It's using associativity of the boolean operators to determine whether to produce a value of 1 or 0, based on whether $enableval == yes.  Originally this blindly assigned a value of 1 to USE_LIGHTGREP, whether --enable or --disable was used.  I tested it and it worked.