lib: fix clang build warnings

lib: fix clang build warnings

Fix the following warning produced by clang 3.5.0:

lib/message.cc:899:4: warning: comparison of constant 64 with expression of type 'notmuch_message_flag_t' (aka '_notmuch_message_flag') is always true [-Wtautological-constant-out-of-range-compare]
        ! NOTMUCH_TEST_BIT (message->lazy_flags, flag))
          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./lib/notmuch-private.h:70:6: note: expanded from macro 'NOTMUCH_TEST_BIT'
    (_NOTMUCH_VALID_BIT(bit) ? !!((val) & (1ull << (bit))) : 0)
     ^~~~~~~~~~~~~~~~~~~~~~~
./lib/notmuch-private.h:68:26: note: expanded from macro '_NOTMUCH_VALID_BIT'
    ((bit) >= 0 && (bit) < CHAR_BIT * sizeof (unsigned long long))
                   ~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~