lib: fix clang build

lib: fix clang build

Long story short, fix build on recent (3.2+) clang.

The long story for posterity follows.

gcc 4.6 added new warnings about structs with greater visibility than
their fields. The warnings were silenced by adjusting visibility in

commit d5523ead90b6be2b07d4af745b8ed9b980a6b9f1
Author: Carl Worth <cworth@cworth.org>
Date:   Wed May 11 13:23:13 2011 -0700

    Mark some structures in the library interface with visibility=default attribute.

Later on,

commit 3b76adf9e2c026dd03b820f4c6eab50e25444113
Author: Austin Clements <amdragon@MIT.EDU>
Date:   Sat Jan 14 19:17:33 2012 -0500

    lib: Add support for automatically excluding tags from queries

changed visibility of struct _notmuch_string_list for the same reason, and

commit 1a53f9f116fa7c460cda3df532be921baaafb082
Author: Mark Walters <markwalters1009@gmail.com>
Date:   Thu Mar 1 22:30:38 2012 +0000

    lib: Add the exclude flag to notmuch_query_search_threads

split the struct _notmuch_string_list and its typedef
notmuch_string_list_t as a way to make a forward declaration for
_notmuch_thread_create().

The subtle difference was that the struct definition now had 'visible'
in it, while the typedef didn't, and it was within the #pragma GCC
visibility push(hidden) block. This went unnoticed, as the then common
versions of clang didn't care about this.

A later change in clang (I did not dig into when this change was
introduced) caused the following error:

CXX  -O2 lib/database.o
In file included from lib/database.cc:21:
In file included from ./lib/database-private.h:33:
./lib/notmuch-private.h:479:8: error: visibility does not match previous declaration
struct visible _notmuch_string_list {
       ^
./lib/notmuch-private.h:67:33: note: expanded from macro 'visible'
                                ^
./lib/notmuch-private.h:52:13: note: previous attribute is here
            ^
1 error generated.
make: *** [lib/database.o] Error 1

This is slightly misleading due to the reference to the #pragma. The
real culprit is the typedef within the #pragma.

We could just add 'visible' to the typedef, or move the typedef
outside of the #pragma, and be done with it, but juggle the
declarations a bit to accommodate moving the typedef back with the
struct, and keep the visibility attribute in one place.

The problem was originally reported by Simonas Kazlauskas
<s@kazlauskas.me> in id:20130418102507.GA23688@godbox but I was only
able to reproduce and investigate now that I upgraded clang.